USB shield at device.

[Sylvia Else]

What to do with the USB cable shield at the device end?

a) Leave it unconnected?

b) Connect it directly to signal ground?

c) Connect to signal ground via some impedance?

Of those, (a) seems to be a common enough choice.

I think (b) is generally regarded as a mistake, since it makes the
shield an antenna for whatever potential EMI exists in the signal ground.

I've seen (c) suggested with a 1 Meg resistor paralleled by a capacitor
in the nano-Farad region. I don't understand that at all.

Since there seems much uncertainty around this, I thought I seek yet
more opinions here.

Sylvia.

[John Larkin]

On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
<syl...@not.at.this.address> wrote:

>What to do with the USB cable shield at the device end?
>
>a) Leave it unconnected?
>
>b) Connect it directly to signal ground?
>
>c) Connect to signal ground via some impedance?
>
>Of those, (a) seems to be a common enough choice.
>
>I think (b) is generally regarded as a mistake, since it makes the
>shield an antenna for whatever potential EMI exists in the signal ground.

Ground the shield at both ends, to the PCB ground plane (which should
also be the case ground.)

>
>I've seen (c) suggested with a 1 Meg resistor paralleled by a capacitor
>in the nano-Farad region. I don't understand that at all.

That's because it makes no sense. If you want a good DC connection and
a good AC connection, just connect them.


--

John Larkin Highland Technology, Inc

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

[Sylvia Else]

On 23/10/2014 2:39 PM, John Larkin wrote:
> On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
> <syl...@not.at.this.address> wrote:
>
>> What to do with the USB cable shield at the device end?
>>
>> a) Leave it unconnected?
>>
>> b) Connect it directly to signal ground?
>>
>> c) Connect to signal ground via some impedance?
>>
>> Of those, (a) seems to be a common enough choice.
>>
>> I think (b) is generally regarded as a mistake, since it makes the
>> shield an antenna for whatever potential EMI exists in the signal ground.
>
> Ground the shield at both ends, to the PCB ground plane (which should
> also be the case ground.)

I don't have the luxury of a PCB ground plane, and I suspect that's true
of a good many USB devices. In any case, the USB data signals reach the
microcontroller within 5mm from the USB receptacle, and none of the rest
of the circuit operates at high frequencies. The case is (OK, will be)
plastic, which is also pretty much the norm.

This is not a commercial project, and I can get away with any amount of
EMI that doesn't cause me problems directly, or result in a regulator
arriving at my door. Still, I like to do things right if possible.

Sylvia.

[upsid...@downunder.com]

On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
<syl...@not.at.this.address> wrote:

>What to do with the USB cable shield at the device end?
>
>a) Leave it unconnected?

This is good against electrostatically connected interference.

>b) Connect it directly to signal ground?

This is a bad idea, if the cable ends are connected to different
grounds, creating different ground potentials and hence large currents
to equalize the potentials. The current is often at mains frequency
and harmonics.

>c) Connect to signal ground via some impedance?
>
>Of those, (a) seems to be a common enough choice.
>
>I think (b) is generally regarded as a mistake, since it makes the
>shield an antenna for whatever potential EMI exists in the signal ground.
>
>I've seen (c) suggested with a 1 Meg resistor paralleled by a capacitor
>in the nano-Farad region. I don't understand that at all.

Apparently the resistor is there to equalize any static electricity
build up if one or both ends are floating.

The small capacitor at least breaks any mains frequency interference
loops.

If the device has a metallic enclosure, the cable shield should be
connected to the enclosure directly or through an impedance, not to
the signal ground.

[John Larkin]

On Thu, 23 Oct 2014 14:46:24 +1100, Sylvia Else

Well, just connect the cable shield to the uP ground and the cable
ground wire.

[Jasen Betts]

On 2014-10-23, Sylvia Else <syl...@not.at.this.address> wrote:
> What to do with the USB cable shield at the device end?
>
> a) Leave it unconnected?
>
> b) Connect it directly to signal ground?
>
> c) Connect to signal ground via some impedance?

Connect it to device shield. (in other words option a)


--
umop apisdn

[George Herold]

I'll sometimes put in two pads for a jumper. Then you can connect, play around after the fact. (Short/ open or an R//C.)

George H.

[WangoTango]

In article <carb6n...@mid.individual.net>,
syl...@not.at.this.address says...

I always use (a).

[John Larkin]

On Thu, 23 Oct 2014 07:27:48 +0300, upsid...@downunder.com wrote:

>On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
><syl...@not.at.this.address> wrote:
>
>>What to do with the USB cable shield at the device end?
>>
>>a) Leave it unconnected?
>
>This is good against electrostatically connected interference.
>
>>b) Connect it directly to signal ground?
>
>This is a bad idea, if the cable ends are connected to different
>grounds, creating different ground potentials and hence large currents
>to equalize the potentials. The current is often at mains frequency
>and harmonics.

We *want* to equalize those potentials! The shield helps do that.

When you connect a signal generator to an oscilloscope with a BNC-BNC
coax cable, do you strip out the shield on the scope end of the cable?

--

John Larkin Highland Technology, Inc

picosecond timing precision measurement

[upsid...@downunder.com]

On Fri, 24 Oct 2014 11:44:35 -0700, John Larkin
<jla...@highlandtechnology.com> wrote:

>On Thu, 23 Oct 2014 07:27:48 +0300, upsid...@downunder.com wrote:
>
>>On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
>><syl...@not.at.this.address> wrote:
>>
>>>What to do with the USB cable shield at the device end?
>>>
>>>a) Leave it unconnected?
>>
>>This is good against electrostatically connected interference.
>>
>>>b) Connect it directly to signal ground?
>>
>>This is a bad idea, if the cable ends are connected to different
>>grounds, creating different ground potentials and hence large currents
>>to equalize the potentials. The current is often at mains frequency
>>and harmonics.
>
>We *want* to equalize those potentials! The shield helps do that.

This applies both to 120/240 V antiphase feeds in the US as well as to
three phase 230/400 V systems in the rest of the world.

The principal idea in both systems is that with _balanced_ loads, the
neutral wire current cancels and the neutral wire potential is the
same as the ground electrode potential all over the place.

Unfortunately this is not so in the real situation.

Assume an apartment building with two stairs with separate feeders
along the stairs. There are going to be unequal voltage drops in the
phase as well as neutral conductor at both stairs.

At some higher floor, measure the mains neutral/ground connector
potential against the potential of the same floor of the adjacent
stairs. With absolutely equal and symmetrical load, the neutral/ground
potential would be equal.

In practice, this is not the case. Connect an unisolated RS-232 or
USB cable between the stairs and the shields might quite well be 1-5
Vrms at different potential.

Guess what is going to happen if you connect the shields together ?

With a bad unbalance load between the stairs feeders, a huge amount
(several amperes) may flow from staircase 1 to staircase 2 neutral
through your signal cable shield, causing _a_lot_ havoc to your
communication.

For this reason, one should avoid connecting cable shields together.

>When you connect a signal generator to an oscilloscope with a BNC-BNC
>coax cable, do you strip out the shield on the scope end of the cable?

As long as you are using a single mains outlet to feed a grounded
extension cable for these two devices, the device ground potentials
should be within a few millivolts.

[Jim Thompson]

I experienced a 60V !!! differential between two GenRad buildings
about 100' apart, so I used a transformer at one end of the coax
(Manchester encoded) to avoid ground currents.

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

[upsid...@downunder.com]

Not a surprise at all. In some countries the electric wiring
conventions can be quite "relaxed".

These days, I prefer to use 10/100/1000BaseT Ethernet simply because
of the simple (standard) galvanic isolation.

However, due to lighting protection and hence the different ground
potential rise I would recommend using fibre links between buildings.

I have been working with industrial applications all over the world
(including the tropics with high lightning rates), first my
recommendations about isolation was deemed too expensive, but after a
few weeks or months, my recommendations were accepted without
objections :-)

[Klaus Kragelund]

USB data signals d+ and d- has CM range 0.8V to 2.5V and diff level of 200mV, so it has some protection from ground loops, but not much. Doesn't the USB standard define maximum cable length so a connection to ground planes on each side is allowed since they assume small CM voltages?

Cheers

Klaus

[Jim Thompson]

Same here, now. My 60V case was ~35 years ago.

>
>I have been working with industrial applications all over the world
>(including the tropics with high lightning rates), first my
>recommendations about isolation was deemed too expensive, but after a
>few weeks or months, my recommendations were accepted without
>objections :-)

[Lasse Langwadt Christensen]

maximum length is 5 meters. There is a ground wire in the cable so the grounds will always be connected, it is whether the chassis are connected


-Lasse

[upsid...@downunder.com]

On Fri, 24 Oct 2014 13:39:52 -0700, Jim Thompson

In practice, where did you measure those figures ?

If you had a 230/400 Vac feed and run it through a six pulse rectifier
and have two capacitors across the DC link, the mid-point potential
varies by 30-40 Vac from nominal ground potential.

For a US 480 V feed you are close to 60 Vrms midpoint to ground
potential.

[John Larkin]

On Fri, 24 Oct 2014 23:34:37 +0300, upsid...@downunder.com wrote:

Well, answer the question: go you break the shield in BNC cables?



We don't connect power neutrals to circuit common or to chassis; we
connect circuits and boxes to GROUND.

If the commons of two USB units have, for some reason, some voltage
difference, that difference will be applied across the super-skinny
ground wire (pin 4) inside the cable. If that wimpy wire can't
equalize the difference, we're going to get nasty data errors and
maybe damage. The cable shield is a much lower resistance than that
skinny ground wire; wouldn't we rather use the shield in parallel with
that tiny wire to short the chassis potentials together?

Why would you WANT the shield, at the device end, to be a different
potential from the ground wire? That's terrible for DC, AC, and RF.

The power outlet grounds on opposite sides of my office are about 50
mV RMS apart at 60 Hz, with a couple hundred more mV of fast trash.
USB is not a long-distance bus.

[John Larkin]

Right. You don't run USB between buildings.

I have an outdoor webcam at our cabin that is 75 feet from the CPU,
with two active USB cables and two passive cables strung together.
Works fine, but the cam is not grounded.

Our philosophy: ground everything.

[Jim Thompson]

Local "ground" to local "ground" and it _was_ 60Vrms... fried the
first cable I plugged in, with an enormous flash :-(

[Phil Allison]

upsid...@downunder.com wrote:

>
> This applies both to 120/240 V antiphase feeds in the US as well as to
> three phase 230/400 V systems in the rest of the world.
>
> The principal idea in both systems is that with _balanced_ loads, the
> neutral wire current cancels and the neutral wire potential is the
> same as the ground electrode potential all over the place.

** No one who is not insane ever assumes that nonsense to be the case.

Neutral current is assumed to be either the same or somewhat more than the maximum current in one phase. Max voltage drops along cables can be computed from this.

> Assume an apartment building with two stairs with separate feeders
> along the stairs. There are going to be unequal voltage drops in the
> phase as well as neutral conductor at both stairs.
>
> At some higher floor, measure the mains neutral/ground connector
> potential against the potential of the same floor of the adjacent
> stairs. With absolutely equal and symmetrical load, the neutral/ground
> potential would be equal.
>
> In practice, this is not the case. Connect an unisolated RS-232 or
> USB cable between the stairs and the shields might quite well be 1-5
> Vrms at different potential.

** What halluncinary drugs are you on today ??

No-one connects signal grounds to the damn NEUTRAL !!!

Doing that would be CRIMINAL !!

>
> Guess what is going to happen if you connect the shields together ?

** Nothing - cos the earth conductor carries virtually no current and so has no voltage drops.

> With a bad unbalance load between the stairs feeders, a huge amount
> (several amperes) may flow from staircase 1 to staircase 2 neutral
> through your signal cable shield, causing _a_lot_ havoc to your
> communication.

** What halluncinary drugs are you on today ??

No-one connects signal grounds to the damn NEUTRAL !!!

Doing that would be CRIMINAL !!

>
> For this reason, one should avoid connecting cable shields together.
>

** More insane crap.

You need kicking off this NG pal.

> >When you connect a signal generator to an oscilloscope with a BNC-BNC
> >coax cable, do you strip out the shield on the scope end of the cable?
>
> As long as you are using a single mains outlet to feed a grounded
> extension cable for these two devices, the device ground potentials
> should be within a few millivolts.

** Same goes for any AC power outlet in the same premises.

Neutral current is not involved unless there is a dangerous fault in the AC supply wiring.


.... Phil

[Les Cargill]

John Larkin wrote:
> On Thu, 23 Oct 2014 07:27:48 +0300, upsid...@downunder.com wrote:
>
>> On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
>> <syl...@not.at.this.address> wrote:
>>
>>> What to do with the USB cable shield at the device end?
>>>
>>> a) Leave it unconnected?
>>
>> This is good against electrostatically connected interference.
>>
>>> b) Connect it directly to signal ground?
>>
>> This is a bad idea, if the cable ends are connected to different
>> grounds, creating different ground potentials and hence large currents
>> to equalize the potentials. The current is often at mains frequency
>> and harmonics.
>
> We *want* to equalize those potentials! The shield helps do that.
>
> When you connect a signal generator to an oscilloscope with a BNC-BNC
> coax cable, do you strip out the shield on the scope end of the cable?
>
>

With the BNC, SFAIK that's a single-ended connection - the lead is in
the middle, the coax is the return.

With an XLR, you may want to float ( open ) one end of the shield for a
balanced connection. There may be a case ground potential difference
from box 1 to box 2.

The only question is which model works best for USB.

http://www.usbpinout.net/

I see a D+/D- pair. And a ground. Looks balanced to me. You may want
the shield connected only at one end.

Given all this, USB cables are not usually that long, so the chance of
any case ground potential is pretty low. But you never know.

--
Les Cargill

[Les Cargill]

Optical is even better.

> However, due to lighting protection and hence the different ground
> potential rise I would recommend using fibre links between buildings.
>
> I have been working with industrial applications all over the world
> (including the tropics with high lightning rates), first my
> recommendations about isolation was deemed too expensive, but after a
> few weeks or months, my recommendations were accepted without
> objections :-)
>

--
Les Cargill

[Les Cargill]

I would not assume signal ground is the same as case ground.

I've looked; I don't see any "ground lift" adapters for USB so it's
maybe not something that comes up.

--
Les Cargill

[John Larkin]

You can buy USB isolators.

--

John Larkin Highland Technology, Inc

[John Larkin]

On Fri, 24 Oct 2014 22:59:44 -0500, Les Cargill
<lcarg...@comcast.com> wrote:

>John Larkin wrote:
>> On Thu, 23 Oct 2014 07:27:48 +0300, upsid...@downunder.com wrote:
>>
>>> On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
>>> <syl...@not.at.this.address> wrote:
>>>
>>>> What to do with the USB cable shield at the device end?
>>>>
>>>> a) Leave it unconnected?
>>>
>>> This is good against electrostatically connected interference.
>>>
>>>> b) Connect it directly to signal ground?
>>>
>>> This is a bad idea, if the cable ends are connected to different
>>> grounds, creating different ground potentials and hence large currents
>>> to equalize the potentials. The current is often at mains frequency
>>> and harmonics.
>>
>> We *want* to equalize those potentials! The shield helps do that.
>>
>> When you connect a signal generator to an oscilloscope with a BNC-BNC
>> coax cable, do you strip out the shield on the scope end of the cable?
>>
>>
>
>
>With the BNC, SFAIK that's a single-ended connection - the lead is in
>the middle, the coax is the return.
>
>With an XLR, you may want to float ( open ) one end of the shield for a
>balanced connection. There may be a case ground potential difference
>from box 1 to box 2.

I can't see why. That allows the unconnected shield to be a nice
antenna for hash or RF.

For minimum common-mode on the differential signal, it's best to
connect the shield at both ends. That reduces the noted potential
difference.

>
>The only question is which model works best for USB.
>
>http://www.usbpinout.net/
>
>I see a D+/D- pair. And a ground. Looks balanced to me. You may want
>the shield connected only at one end.

In that case, disconnect the ground wire too.

--

John Larkin Highland Technology, Inc

[jrwal...@gmail.com]

On Saturday, 25 October 2014 06:17:34 UTC+1, John Larkin wrote:

> >I see a D+/D- pair. And a ground. Looks balanced to me. You may want
> >the shield connected only at one end.
>
> In that case, disconnect the ground wire too.

Some aplication notes even recommend this - in the form of a ferrite
bead in the USB ground connection. however,it seems that such
ferrites are a common cause of USB compliance failures as they also
slow down certain state transitions which use common mode signalling
as well as the differential mode which is used for ordinary data
transfer.

For getting through EMC compliance testing, bonding the grounds
together as solidly as possible works well.

John

[upsid...@downunder.com]

On Fri, 24 Oct 2014 15:23:48 -0700, John Larkin

Take a look at any 10Base2 Thin-Ethernet card. The connector (cable
shield) is isolated from the metallic card frame and the computer
frame. Also when building 10Base2 network, the cable shield must be
grounded at exactly one point, usually at the other terminator. An
unisolated expansion barrel connector touching some grounded support
structure may ruin the network, now that we have two grounding points
and hence ground loop currents.

>We don't connect power neutrals to circuit common or to chassis; we
>connect circuits and boxes to GROUND.

As far as I understand, in the US, the ground and neutral are
connected together at the service entry as well as connected to a
physical grounding electrode.

As long as you connect two devices powered from the same service
entry, the ground potential of the data cable shield ends is the same.

A power supply connected through an EMC filter will spew interference
current into the ground connector through the Y-capacitor, thus
causing unequal voltage drops in different ground conductors, thus,
there is some high frequency interference current flowing in the data
cable shield.

[upsid...@downunder.com]

Not directly of course, but look at the big picture.

Take a look at en.wikipedia.org/wiki/Earthing_system and especially
TN-C-S

Do you expect that the Neutral (N) and protective ground (PE) wires
would run the whole way back to the power plant. That would be a true
TN-S system with total of 5 wires. In practice the distribution system
is some form of TN-C-S, in which the PEN (combined protective earth
and neutral starts at the distribution transformer star point. This
PEN conductor is split into PE and N at some point towards the
customer.

This splitting point could be at the main distribution panel, in any
sub distribution panel or in apartment specific distribution panel. It
would not at all be strange to have a common PEN conductor in the
raiser cables in two separate stairs and hence different ground
potential between stairs.

Some older regulation allow the PEN to PE and N in each mains socket.
Thus, the equipment mains cord, is the only -S part in TN-C-S

If the equipment signal ground or shield are connected to the
equipment local PE ground, there may be ground potential differences
between devices even in the same room, same apartment and at least
between buildings.

[Phil Allison]

Jerk Larkin wrote:

>
> >With an XLR, you may want to float ( open ) one end of the shield for a
> >balanced connection. There may be a case ground potential difference
> >from box 1 to box 2.
>
> I can't see why. That allows the unconnected shield to be a nice
> antenna for hash or RF.
>

** Amazing how most of the worlds's microphones manage to work so perfectly - ain't it ??

FYI:

Connecting a cable's shield *directly* to metalwork at the signal receiving end diverts RF energy harmlessly away - this has been standard practice with audio and RF devices for centuries.

Connecting the metalwork of various audio devices that are all linked to a common safety earth can sometimes result in ( low level) hum loops - even with all balanced set ups.

Those who do *not* deal with a particular area of technology have no right to pontificate about it.

Even if they imagine themselves to be god.



... Phil

[Phil Allison]

upsid...@downunder.com wrote:



** What halluncinary drugs are you on today ??

No-one connects signal grounds to the damn NEUTRAL !!!

Doing that would be CRIMINAL !!

Not that anyone should put that past a raving nutter like you.

Piss off and die.



... Phil

[Michael A. Terrell]

John Larkin wrote:
>
> I can't see why. That allows the unconnected shield to be a nice
> antenna for hash or RF.
>
> For minimum common-mode on the differential signal, it's best to
> connect the shield at both ends. That reduces the noted potential
> difference.

Try that at a transmitter site. You can get enough RF current to
burn the shield away.

--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

[John Larkin]

10b2 is not USB. It is transformer isolated, and was notoriously
unreliable.

>
>>We don't connect power neutrals to circuit common or to chassis; we
>>connect circuits and boxes to GROUND.
>
>As far as I understand, in the US, the ground and neutral are
>connected together at the service entry as well as connected to a
>physical grounding electrode.

At one point. But there should be no current in the ground wires,
hence no ground-ground voltage differences. And difference will
probably come from magnetic coupling from line currents, which will be
small and relatively high impedance. The shield of a short USB cable,
connected at both ends, can significantly short out, and reduce, and
ground-ground voltages.

>
>As long as you connect two devices powered from the same service
>entry, the ground potential of the data cable shield ends is the same.

There's also high frequency stuff, like spikes and ambient RF. A cable
with one end connected is a monopole antenna. The open end can have
serious amounts of RF on the shield, especially at resonant
frequencies, and that is coupled hard to the four wires inside. Ground
the shield at both ands, and the RF goes away.

>
>A power supply connected through an EMC filter will spew interference
>current into the ground connector through the Y-capacitor, thus
>causing unequal voltage drops in different ground conductors, thus,
>there is some high frequency interference current flowing in the data
>cable shield.

All the more reason to ground the shield on both ends.

There's yet another reason to ground the shields. The shield forms a
transmission-line transformer with the wires inside it. So delta-v
between the shield ends becomes (at higher frequencies) delta-v in the
wires, forming a common-mode-rejection transformer for free.

Ground on both ends.

--

John Larkin Highland Technology, Inc

[k...@attt.bizz]

They exist. I use an isolated hub for all of my USB attached lab
equipment, including (especially) RS232 adapters and sound "cards".

[k...@attt.bizz]

On Sat, 25 Oct 2014 12:26:50 +0300, upsid...@downunder.com wrote:

>On Fri, 24 Oct 2014 15:23:48 -0700, John Larkin

<...>

>>We don't connect power neutrals to circuit common or to chassis; we
>>connect circuits and boxes to GROUND.
>
>As far as I understand, in the US, the ground and neutral are
>connected together at the service entry as well as connected to a
>physical grounding electrode.

Not quite. They are tied together at the service entry and the
grounding electrode is in tied at that point. They are also tied
together at the transformer (center tap) feeding the building.

>As long as you connect two devices powered from the same service
>entry, the ground potential of the data cable shield ends is the same.

There can still be a significant difference between the two,
particularly under fault conditions. The AC difference can be
substantial (and ruin your EMI compliance day).

[k...@attt.bizz]

On Fri, 24 Oct 2014 22:17:25 -0700, John Larkin
<jla...@highlandtechnology.com> wrote:

>On Fri, 24 Oct 2014 22:59:44 -0500, Les Cargill
><lcarg...@comcast.com> wrote:
>
>>John Larkin wrote:
>>> On Thu, 23 Oct 2014 07:27:48 +0300, upsid...@downunder.com wrote:
>>>
>>>> On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
>>>> <syl...@not.at.this.address> wrote:
>>>>
>>>>> What to do with the USB cable shield at the device end?
>>>>>
>>>>> a) Leave it unconnected?
>>>>
>>>> This is good against electrostatically connected interference.
>>>>
>>>>> b) Connect it directly to signal ground?
>>>>
>>>> This is a bad idea, if the cable ends are connected to different
>>>> grounds, creating different ground potentials and hence large currents
>>>> to equalize the potentials. The current is often at mains frequency
>>>> and harmonics.
>>>
>>> We *want* to equalize those potentials! The shield helps do that.
>>>
>>> When you connect a signal generator to an oscilloscope with a BNC-BNC
>>> coax cable, do you strip out the shield on the scope end of the cable?
>>>
>>>
>>
>>
>>With the BNC, SFAIK that's a single-ended connection - the lead is in
>>the middle, the coax is the return.
>>
>>With an XLR, you may want to float ( open ) one end of the shield for a
>>balanced connection. There may be a case ground potential difference
>>from box 1 to box 2.

It's a balanced connection anyway. It's a three (or four) wire
connection. The problem is the length of the cable means that it's
possible for it to cross power domains.

>I can't see why. That allows the unconnected shield to be a nice
>antenna for hash or RF.

Because there may be 50-100V on the shield, and not just some
capacitive coupling (I've seen it). Connect the two together and
you'll see sparks and smell smoke. XLRs are generally connected
through isolation transformers. For a reason.

>For minimum common-mode on the differential signal, it's best to
>connect the shield at both ends. That reduces the noted potential
>difference.

But safety (of both the user and the equipment) trumps making your EMI
compliance easy.

>>The only question is which model works best for USB.
>>
>>http://www.usbpinout.net/
>>
>>I see a D+/D- pair. And a ground. Looks balanced to me. You may want
>>the shield connected only at one end.
>
>In that case, disconnect the ground wire too.

You snipped the critical point. There is little chance of a dangerous
ground differential, so ground both ends.

[k...@attt.bizz]

On Sat, 25 Oct 2014 08:48:39 -0400, "Michael A. Terrell"
<mike.t...@earthlink.net> wrote:

>
>John Larkin wrote:
>>
>> I can't see why. That allows the unconnected shield to be a nice
>> antenna for hash or RF.
>>
>> For minimum common-mode on the differential signal, it's best to
>> connect the shield at both ends. That reduces the noted potential
>> difference.
>
>
> Try that at a transmitter site. You can get enough RF current to
>burn the shield away.

That solves the problem. ;-)

[Michael A. Terrell]

Not if the entire cable goes up in flames. :)

[John Larkin]

On Sat, 25 Oct 2014 08:48:39 -0400, "Michael A. Terrell"
<mike.t...@earthlink.net> wrote:

>

>John Larkin wrote:
>>
>> I can't see why. That allows the unconnected shield to be a nice
>> antenna for hash or RF.
>>
>> For minimum common-mode on the differential signal, it's best to
>> connect the shield at both ends. That reduces the noted potential
>> difference.
>
>
> Try that at a transmitter site. You can get enough RF current to
>burn the shield away.

If you break the shield at one end, you convert that huge current into
a huge voltage... coupled directly to the wires inside the shield. Fun
things will then happen inside the box.

Few of us operate USB gear inside a big transmitter. If we did, the
best arrangement would be to ground the shield at both ends and add
clamp-on ferrites to reduce the current in the shield.

[John Larkin]

If there's low impedance 50-100 volts between power outlet grounds,
the building wiring is defective and dangerous. And it will blow out
the USB electronics.

If that 50-100 volts is high impedance, like from capacitive coupling,
a continuous shield will short it out and protect the electronics. As
it also shunts RF to the chassis on both ends.

Even transformer coupled audio signals have some finite common-mode
rejection at the receive end. An open shield is an antenna that jams
cm voltages onto all the wires inside the cable.

>
>>For minimum common-mode on the differential signal, it's best to
>>connect the shield at both ends. That reduces the noted potential
>>difference.
>
>But safety (of both the user and the equipment) trumps making your EMI
>compliance easy.

How is an open shield safer than a continuous one? Which is safer for
the USB target chip:

1. Shield connects first and zeroes out any ESD, ground loop voltage,
and EMI

or

2. Let the signal wires handle all of that.


Can you buy a USB cable, or an XLR audio cable, or a shielded RS232 or
CAT6 that has the shield connected at just one end? How about a
BNC-BNC coax that has a disconnected shield on one end?

>
>>>The only question is which model works best for USB.
>>>
>>>http://www.usbpinout.net/
>>>
>>>I see a D+/D- pair. And a ground. Looks balanced to me. You may want
>>>the shield connected only at one end.
>>
>>In that case, disconnect the ground wire too.
>
>You snipped the critical point. There is little chance of a dangerous
>ground differential, so ground both ends.

Right. Ground the shield at both ends.

[k...@attt.bizz]

On Sat, 25 Oct 2014 10:13:04 -0700, John Larkin

That discussion was about XLR (audio). As I noted, USB is a different
argument completely.

>If that 50-100 volts is high impedance, like from capacitive coupling,
>a continuous shield will short it out and protect the electronics. As
>it also shunts RF to the chassis on both ends.
>
>Even transformer coupled audio signals have some finite common-mode
>rejection at the receive end. An open shield is an antenna that jams
>cm voltages onto all the wires inside the cable.

That's why they're transformer coupled.

>>
>>>For minimum common-mode on the differential signal, it's best to
>>>connect the shield at both ends. That reduces the noted potential
>>>difference.
>>
>>But safety (of both the user and the equipment) trumps making your EMI
>>compliance easy.
>
>How is an open shield safer than a continuous one? Which is safer for
>the USB target chip:

You're confusing USB with audio (XLR). Different problems, different
solutions.

>1. Shield connects first and zeroes out any ESD, ground loop voltage,
>and EMI
>
>or
>
>2. Let the signal wires handle all of that.
>
>
>Can you buy a USB cable, or an XLR audio cable, or a shielded RS232 or
>CAT6 that has the shield connected at just one end?

XLR cables are usually grounded only at one end. It's common to
unground the shield on RCA cables, too (particularly at low-level
sources, like turn tables).

>How about a
>BNC-BNC coax that has a disconnected shield on one end?

Is BNC differential? What are you trying to accomplish. A solution
isn't very useful without a state problem.

>
>>
>>>>The only question is which model works best for USB.
>>>>
>>>>http://www.usbpinout.net/
>>>>
>>>>I see a D+/D- pair. And a ground. Looks balanced to me. You may want
>>>>the shield connected only at one end.
>>>
>>>In that case, disconnect the ground wire too.
>>
>>You snipped the critical point. There is little chance of a dangerous
>>ground differential, so ground both ends.
>
>Right. Ground the shield at both ends.

That's a good idea, unless it causes more serious problems. It often
does. One size does *not* fit all. I generally ground chassis
through 1206 sorts of devices so I have "knobs" to turn if testing
doesn't go well.

[Michael A. Terrell]

John Larkin wrote:
>
> On Sat, 25 Oct 2014 08:48:39 -0400, "Michael A. Terrell"
> <mike.t...@earthlink.net> wrote:
>
> >
> >John Larkin wrote:
> >>
> >> I can't see why. That allows the unconnected shield to be a nice
> >> antenna for hash or RF.
> >>
> >> For minimum common-mode on the differential signal, it's best to
> >> connect the shield at both ends. That reduces the noted potential
> >> difference.
> >
> >
> > Try that at a transmitter site. You can get enough RF current to
> >burn the shield away.
>
> If you break the shield at one end, you convert that huge current into
> a huge voltage... coupled directly to the wires inside the shield. Fun
> things will then happen inside the box.
>
> Few of us operate USB gear inside a big transmitter. If we did, the
> best arrangement would be to ground the shield at both ends and add
> clamp-on ferrites to reduce the current in the shield.

Not inside a transmitter, but at a transmitter site where you use
1/8" thick, 6" wide copper straps for grounds.

<https://www.facebook.com/photo.php?fbid=10203012833103338&set=gm.775884959139820&type=1>
shows a nice burn mark on a large copper feedline from a transmitter.

[Michael A. Terrell]

John Larkin wrote:
>
> Even transformer coupled audio signals have some finite common-mode
> rejection at the receive end. An open shield is an antenna that jams
> cm voltages onto all the wires inside the cable.

You connect the shield at one end. The input. Otherwise you can have
all kinds of problems.

[Les Cargill]

And so they do - "USB isolators". Spendy, too.

--
Les Cargill

[Les Cargill]

It's connected at one end. That should be enough to drain off EMI.
Yeah, you'll have residual emitted hash from the shield, but
presumably that's less bad than massive hum because of
case potential difference.

I think the term of art is "telescoping ground".

Remember, this only matters if there's enough potential between the two
case grounds are a problem. Ideally, this is never the case. You can
also put a filter in series on the ground line.

For USB, the frequencies may also be far enough apart that it doesn't
matter.

> For minimum common-mode on the differential signal, it's best to
> connect the shield at both ends. That reduces the noted potential
> difference.
>
>
>>
>> The only question is which model works best for USB.
>>
>> http://www.usbpinout.net/
>>
>> I see a D+/D- pair. And a ground. Looks balanced to me. You may want
>> the shield connected only at one end.
>
> In that case, disconnect the ground wire too.
>
>

Signal ground should(?) probably be isolated from case ground.



--
Les Cargill

[k...@attt.bizz]

On Sat, 25 Oct 2014 12:31:51 -0500, Les Cargill

Yes, I spent $400 (of the boss' money) on the isolated hub. They
aren't commonly needed so, of course, command a premium. They're
usually built a lot better than the ordinary consumer crap, though.

[rickman]

On 10/22/2014 11:46 PM, Sylvia Else wrote:

> On 23/10/2014 2:39 PM, John Larkin wrote:
>> On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else
>> <syl...@not.at.this.address> wrote:
>>
>>> What to do with the USB cable shield at the device end?
>>>
>>> a) Leave it unconnected?
>>>

>>> b) Connect it directly to signal ground?
>>>

>>> c) Connect to signal ground via some impedance?
>>>
>>> Of those, (a) seems to be a common enough choice.
>>>
>>> I think (b) is generally regarded as a mistake, since it makes the
>>> shield an antenna for whatever potential EMI exists in the signal
>>> ground.
>>
>> Ground the shield at both ends, to the PCB ground plane (which should
>> also be the case ground.)
>
> I don't have the luxury of a PCB ground plane, and I suspect that's true
> of a good many USB devices. In any case, the USB data signals reach the
> microcontroller within 5mm from the USB receptacle, and none of the rest
> of the circuit operates at high frequencies. The case is (OK, will be)
> plastic, which is also pretty much the norm.
>
> This is not a commercial project, and I can get away with any amount of
> EMI that doesn't cause me problems directly, or result in a regulator
> arriving at my door. Still, I like to do things right if possible.

The shield is not there to serve as a conductor in the cable. It is a
shield to keep EMI from radiating or from entering the cable. It works
by being held at ground potential.

The problem with connecting the power ground and the shield ground at
both ends of the cable is that this creates a ground loop. Whether or
not the ground loop is a problem depends on the details of how they are
connected. It is customary for the shield to be connected at just one
end of the cable. I expect in 99.9% of installations it makes no
effective difference.

--

Rick

[rickman]

On 10/24/2014 8:41 PM, Phil Allison wrote:

> upsid...@downunder.com wrote:
>
>>
>> This applies both to 120/240 V antiphase feeds in the US as well as to
>> three phase 230/400 V systems in the rest of the world.
>>
>> The principal idea in both systems is that with _balanced_ loads, the
>> neutral wire current cancels and the neutral wire potential is the
>> same as the ground electrode potential all over the place.
>

> ** No one who is not insane ever assumes that nonsense to be the case.
>
> Neutral current is assumed to be either the same or somewhat more than the maximum current in one phase. Max voltage drops along cables can be computed from this.
>
>

>> Assume an apartment building with two stairs with separate feeders
>> along the stairs. There are going to be unequal voltage drops in the
>> phase as well as neutral conductor at both stairs.
>>
>> At some higher floor, measure the mains neutral/ground connector
>> potential against the potential of the same floor of the adjacent
>> stairs. With absolutely equal and symmetrical load, the neutral/ground
>> potential would be equal.
>>
>> In practice, this is not the case. Connect an unisolated RS-232 or

>> USB cable between the stairs and the shields might quite well be 1-5
>> Vrms at different potential.

>
> ** What halluncinary drugs are you on today ??
>
> No-one connects signal grounds to the damn NEUTRAL !!!
>
> Doing that would be CRIMINAL !!
>
>
>>

>> Guess what is going to happen if you connect the shields together ?
>
>

> ** Nothing - cos the earth conductor carries virtually no current and so has no voltage drops.
>
>

>> With a bad unbalance load between the stairs feeders, a huge amount
>> (several amperes) may flow from staircase 1 to staircase 2 neutral
>> through your signal cable shield, causing _a_lot_ havoc to your
>> communication.
>
>

> ** What halluncinary drugs are you on today ??
>
> No-one connects signal grounds to the damn NEUTRAL !!!
>
> Doing that would be CRIMINAL !!
>
>
>>

>> For this reason, one should avoid connecting cable shields together.
>>

> ** More insane crap.
>
> You need kicking off this NG pal.
>
>

>>> When you connect a signal generator to an oscilloscope with a BNC-BNC
>>> coax cable, do you strip out the shield on the scope end of the cable?
>>

>> As long as you are using a single mains outlet to feed a grounded
>> extension cable for these two devices, the device ground potentials
>> should be within a few millivolts.
>

> ** Same goes for any AC power outlet in the same premises.
>
> Neutral current is not involved unless there is a dangerous fault in the AC supply wiring.

Phil keeps asking about hallucinatory drugs. I guess he has run out and
is looking for a new supply.

--

Rick

[John Larkin]

On Sat, 25 Oct 2014 12:40:24 -0500, Les Cargill

A conductor grounded on one end is also known as a "vertical antenna."
It will resonate at multiple frequencies, and the voltage at the far
end will be jammed directly onto the wires inside the shield/antenna.

A room nowadays is flooded with RF. You can often hear cell phone
bursts getting into PA systems and such. Too many unconnected shields!

>Yeah, you'll have residual emitted hash from the shield, but
>presumably that's less bad than massive hum because of
>case potential difference.

Case potential differences are *reduced* by a continuous shield.
Opening the shield certainly doesn't help the signals inside. If the
grounds are at seriously different voltages, go differential on the
signals. Ground the shield at both ends to reduce the common-mode
voltage.

What about ESD? The shield and connector shell discharge that... if
the shield is connected!

[Les Cargill]

USB is differential signals. So it's at least partially the same argument.

Thing about USB is that you're much less likely to have power from
different service points. You are, however, likely to have wall warts,
and wall warts emit noxious ground potentials sometimes.

Unfortunately, USB is usually cheap consumer stuff, so these things happen.

Had I never been bit by it, I wouldna said nothin'. :) In this case,
I was able to sub in a different power supply.

>> If that 50-100 volts is high impedance, like from capacitive coupling,
>> a continuous shield will short it out and protect the electronics. As
>> it also shunts RF to the chassis on both ends.
>>
>> Even transformer coupled audio signals have some finite common-mode
>> rejection at the receive end. An open shield is an antenna that jams
>> cm voltages onto all the wires inside the cable.
>
> That's why they're transformer coupled.
>

Transformer coupled audio is pretty rare these days.

<snip>

--
Les Cargill

[Les Cargill]

Oh, I understand. Ideally, you never have to think about this but
teh dread god Chaos sometimes plays jokes on us...

> A room nowadays is flooded with RF. You can often hear cell phone
> bursts getting into PA systems and such. Too many unconnected shields!
>

Any hi-Z input is susceptible to cell hash.

>
>> Yeah, you'll have residual emitted hash from the shield, but
>> presumably that's less bad than massive hum because of
>> case potential difference.
>
> Case potential differences are *reduced* by a continuous shield.

... but sometimes in tragic ways...

> Opening the shield certainly doesn't help the signals inside. If the
> grounds are at seriously different voltages, go differential on the
> signals. Ground the shield at both ends to reduce the common-mode
> voltage.
>
> What about ESD? The shield and connector shell discharge that... if
> the shield is connected!
>
>

It's a Hobson's choice - if the problem present is a ground loop, you
take your chances on all the other stuff you mention. Fortunately for
USB, it's a small antenna.

And yes, I had a USB circuit ( not of our design, some cheap Chinese
stuff being used as test equipment ) that had a ground loop on USB
causing poor performance. We replaced the "power supply" for the
offending device and it all worked.

I would not use USB for deployable systems unless I had no other
choice.

--
Les Cargill

[whit3rd]

On Saturday, October 25, 2014 2:26:49 AM UTC-7, upsid...@downunder.com wrote:

> As far as I understand, in the US, the ground and neutral are
> connected together at the service entry as well as connected to a
> physical grounding electrode.
>
> As long as you connect two devices powered from the same service
> entry, the ground potential of the data cable shield ends is the same.

Maybe, but there ARE code exceptions (for isolated power - it seems odd,
but this HAS bit me, and others, on occasion). If the USB device is bus-powered,
usually there's no ground loop created. If the USB device has an AC power
cord, the only safe connection will have some power limit element
in the shield circuit.

RC and L have already been suggested: let me add "fuse" and PTC resistor to the list).

I've seen scorch marks on the USB shield wiring after a "safety test".

[k...@attt.bizz]

On Sat, 25 Oct 2014 13:26:04 -0500, Les Cargill

No, it's not. The goals/priorities are completely different.

>
>Thing about USB is that you're much less likely to have power from
>different service points. You are, however, likely to have wall warts,
>and wall warts emit noxious ground potentials sometimes.

>Unfortunately, USB is usually cheap consumer stuff, so these things happen.

The FCC still cares, though.

>Had I never been bit by it, I wouldna said nothin'. :) In this case,
>I was able to sub in a different power supply.

It? Which "it"?

>>> If that 50-100 volts is high impedance, like from capacitive coupling,
>>> a continuous shield will short it out and protect the electronics. As
>>> it also shunts RF to the chassis on both ends.
>>>
>>> Even transformer coupled audio signals have some finite common-mode
>>> rejection at the receive end. An open shield is an antenna that jams
>>> cm voltages onto all the wires inside the cable.
>>
>> That's why they're transformer coupled.
>>
>
>Transformer coupled audio is pretty rare these days.

Wrong.

[Tom Swift]

John Larkin <jla...@highlandtechnology.com> wrote:

> A conductor grounded on one end is also known as a "vertical antenna."
> It will resonate at multiple frequencies, and the voltage at the far
> end will be jammed directly onto the wires inside the shield/antenna.

A similar thread was on Time-Nuts a while ago with similar conclusions.
The thread starts here:

Noise and non-linear behaviour of ferrite transformers
https://www.mail-archive.com/time...@febo.com/msg66866.html

A relevant post was made by John Miles, KE5FX:

Date: Sun, 20 Jul 2014
From: John Miles
Subject: Re: [time-nuts] Noise and non-linear behaviour of ferrite
transformers

>> All that said, the real hazard with transformers is that people
>> tend to use them to drive unbalanced coax cables with balanced
>> signals. This turns the coax shield into an antenna, at which
>> point you may end up with with more noise and spurs than you had
>> before.

> Could you explain this a little bit more? Because this would be
> exactly what I would like to do.

I often find that when I use coaxial baluns to cut down on ground
loop noise, I end up with more noise and interference than I started
with. Not always, but often enough that I'm leery of them.

Due to skin effect, most signal propagation in a coaxial cable takes
place between the outer surface of the center conductor and the
inner surface of the braid. Ideally, the outer surface of the braid
just underneath the jacket will act like an equipotential shield to
keep external EMI away from the signal path inside the cable.

But that's really only true when the cable connects two devices in
well-shielded enclosures that are themselves at a similar ground
potential.

When you "lift the ground" with a coaxial balun such as an FTB-1-1+,
you can no longer pretend that the coax braid is at ground potential
along its length. From an RF perspective the braid is floating at
one end, which makes it an antenna.

Put another way, a balun will reject common-mode signals in favor of
differential signals. That's fine if you're using it with a twisted
pair or other balanced line (you're probably aware that this is how
UTP Ethernet cables work). RF interference in such a line is picked
up equally by both conductors and rejected by the balun. But a
length of coax cable is as far from a balanced line as you can get.
One conductor is well-shielded, the other has its outer surface
flapping in the breeze. The balun can't tell the difference between
desired signals on the inside surface of the braid and undesired
signals on its outside surface. They both look like
differential-mode signals, relative to the inner conductor.

The same thing happens with instruments that allow you to lift the
ground at their input jacks. Apart from the unwanted-antenna effect,
this is almost always a bad idea because it's very hard to properly
ground the jack's outer shell to the chassis. Few things in EMC are
more important than ground integrity at the point of entry to an
enclosure.

When fighting ground loops, a good first step is to minimize the
loop area if you can. Try plugging your DUT, reference,
instrumentation, and computer into a single power strip. That will
take care of most of your power-line interference problems. Baluns
can help too, but don't be surprised if they don't.

- john, KE5FX
Miles Design LLC

You can search the Time-Nuts archive using this url plus your search
string:

http://www.febo.com/pipermail/time-nuts/ "Your Search String"

There is an amazing amount of valuable information in the archive.

[Tim Williams]

"Michael A. Terrell" <mike.t...@earthlink.net> wrote in message
news:NN2dndJui9lsQNbJ...@earthlink.com...

> You connect the shield at one end. The input. Otherwise you can have
> all kinds of problems.

I can't believe how much misinformation there is, still persisting today.

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

[Phil Allison]

k...@attt.bizz wrote:

>
> >Can you buy a USB cable, or an XLR audio cable, or a shielded RS232 or
> >CAT6 that has the shield connected at just one end?
>
> XLR cables are usually grounded only at one end.

** Only ones used with microphones where there is obviously no option.

In 40 years of working with professional audio, all XLR cables seen have pin 1 linked to the shield at both ends.

(The metal case of the plug itself may or may not be linked to pin 1.)

> It's common to
> unground the shield on RCA cables, too (particularly at low-level
> sources, like turn tables).

** Huh ?? Wot utter nonsense.

RCA leads supply the *only* ground connection to most audio items that use them - cos for the last 30 years or more almost every thing made is Class II or "double insulated" so does not have a safety ground connection.



.... Phil

[Michael A. Terrell]

Tim Williams wrote:
>
> "Michael A. Terrell" <mike.t...@earthlink.net> wrote in message
> news:NN2dndJui9lsQNbJ...@earthlink.com...
> > You connect the shield at one end. The input. Otherwise you can have
> > all kinds of problems.
>
> I can't believe how much misinformation there is, still persisting today.

Who are you talking about? I've had to cut hundreds of connections
to eliminate ground loop problems in studios. Everything run on a single
phase, of 120/208 three phase. This was all balanced, transformer
coupled audio lines with a transformer at each end and inside metal
equipment cases.

[Tim Williams]

"Michael A. Terrell" <mike.t...@earthlink.net> wrote in message

news:XuOdnYLK9YUXt9HJ...@earthlink.com...

>> I can't believe how much misinformation there is, still persisting
>> today.
>
>
> Who are you talking about? I've had to cut hundreds of connections
> to eliminate ground loop problems in studios. Everything run on a single
> phase, of 120/208 three phase. This was all balanced, transformer
> coupled audio lines with a transformer at each end and inside metal
> equipment cases.

Try that with an EMC susceptibility test...

If it was humming even with isolation (and the shield made any difference
despite the supposed isolation), that's their problem. Hey, just because
it costs $10k doesn't mean it was made at all correctly...

...And it doesn't mean that cutting the shield is ever a good idea in
general. Maybe it works in one special case, maybe it works in a few. It
ain't a general rule, and there's good reason, whether you understand it
or not.

[k...@attt.bizz]

On Sat, 25 Oct 2014 17:37:18 -0500, "Tim Williams"
<tiw...@seventransistorlabs.com> wrote:

>"Michael A. Terrell" <mike.t...@earthlink.net> wrote in message
>news:NN2dndJui9lsQNbJ...@earthlink.com...
>> You connect the shield at one end. The input. Otherwise you can have
>> all kinds of problems.
>
>I can't believe how much misinformation there is, still persisting today.

I guess you're too young to remember turn tables.

[k...@attt.bizz]

On Sat, 25 Oct 2014 18:13:14 -0500, "Tim Williams"
<tiw...@seventransistorlabs.com> wrote:

>"Michael A. Terrell" <mike.t...@earthlink.net> wrote in message
>news:XuOdnYLK9YUXt9HJ...@earthlink.com...
>>> I can't believe how much misinformation there is, still persisting
>>> today.
>>
>>
>> Who are you talking about? I've had to cut hundreds of connections
>> to eliminate ground loop problems in studios. Everything run on a single
>> phase, of 120/208 three phase. This was all balanced, transformer
>> coupled audio lines with a transformer at each end and inside metal
>> equipment cases.
>
>Try that with an EMC susceptibility test...

What good is passing your test if the equipment doesn't function?

>If it was humming even with isolation (and the shield made any difference
>despite the supposed isolation), that's their problem. Hey, just because
>it costs $10k doesn't mean it was made at all correctly...

Nonsense.

>...And it doesn't mean that cutting the shield is ever a good idea in
>general. Maybe it works in one special case, maybe it works in a few. It
>ain't a general rule, and there's good reason, whether you understand it
>or not.

General rules aren't universally useful.

[Phil Allison]

Tim Williams wrote:

>
> > Who are you talking about? I've had to cut hundreds of connections
> > to eliminate ground loop problems in studios. Everything run on a single
> > phase, of 120/208 three phase. This was all balanced, transformer
> > coupled audio lines with a transformer at each end and inside metal
> > equipment cases.
>
> Try that with an EMC susceptibility test...

** Pass with flying colours.

Got any clue what a ( high quality, audio line) transformer does to RF ?

> If it was humming even with isolation (and the shield made any difference
> despite the supposed isolation), that's their problem. Hey, just because
> it costs $10k doesn't mean it was made at all correctly...

** The problem is mainly historic and has to do with the internal signal grounding inside the equipment. Because makers often linked XLR shields to various points on PCBs, induced hum current flowing in the shields of cables could result in hum voltages appearing across PCB earth patterns. Breaking the shield at the send end fixed this.

However, linking all XLR pin 1s direct to metalwork at entry and exit points eliminates the possibility. PCB ground can then link to the metalwork at a single point via say a 100ohm resistor and a 100nF cap in parallel.


... Phil





.... Phil

[David Eather]

On Sun, 26 Oct 2014 09:57:06 +1000, Phil Allison <palli...@gmail.com>
wrote:

> Tim Williams wrote:
>
>>
>> > Who are you talking about? I've had to cut hundreds of connections
>> > to eliminate ground loop problems in studios. Everything run on a
>> single
>> > phase, of 120/208 three phase. This was all balanced, transformer
>> > coupled audio lines with a transformer at each end and inside metal
>> > equipment cases.
>>
>> Try that with an EMC susceptibility test...
>
> ** Pass with flying colours.
>
> Got any clue what a ( high quality, audio line) transformer does to RF ?

Wouldn't that be a bit hard to measure? ;-)

[Les Cargill]

k...@attt.bizz wrote:
> On Sat, 25 Oct 2014 13:26:04 -0500, Les Cargill
> <lcarg...@comcast.com> wrote:

<snip>

>> USB is differential signals. So it's at least partially the same argument.
>
> No, it's not. The goals/priorities are completely different.

Ground loops is ground loops.

>>
>> Thing about USB is that you're much less likely to have power from
>> different service points. You are, however, likely to have wall warts,
>> and wall warts emit noxious ground potentials sometimes.
>
>> Unfortunately, USB is usually cheap consumer stuff, so these things happen.
>
> The FCC still cares, though.
>

So an open ground on one end will cause violation of FCC? Might cause one?

>> Had I never been bit by it, I wouldna said nothin'. :) In this case,
>> I was able to sub in a different power supply.
>
> It? Which "it"?
>

Have a ground loop on USB cause bad performance.

<snip>

>> Transformer coupled audio is pretty rare these days.
>
> Wrong.
>

???

Everybody uses that same 4-transistor circuit Mackie started in the
'90s, outside of upmarket stuff. Just the transformer part cost alone
for a Hammond 140NEX is $50ish bucks...

I haven't seen anything new that was xformer-coupled in a long time.

--
Les Cargill

[upsid...@downunder.com]

On Sat, 25 Oct 2014 10:59:28 -0700, John Larkin
<jla...@highlandtechnology.com> wrote:

>>> I can't see why. That allows the unconnected shield to be a nice
>>> antenna for hash or RF.
>>>
>>
>>It's connected at one end. That should be enough to drain off EMI.
>
>A conductor grounded on one end is also known as a "vertical antenna."
>It will resonate at multiple frequencies, and the voltage at the far
>end will be jammed directly onto the wires inside the shield/antenna.

A 5 m long quarter wave unipolar antenna will have the lowest
resonance at 15 MHz as well as the 3/4 wave resonance at 45 MHz and
5/4 resonance at 75 MHz. At these frequencies the antenna is resistive
and capable of dumping some real power into the losses at the "load"
grounded end.

At other frequencies, the antenna is more or less reactive and
re-radiates more or less of the captured power immediately to the
surrounding space, without flowing and dissipated into the load.

Even with a resonant antenna, the impedance level along the antenna is
not constant. The current is largest and voltage smallest at the load
end (i.e. low impedance), while at the open end, the current is very
low and voltage high i.e. high impedance of several kilo-ohms.

The USB twisted pair transmission line has an impedance of about 100
ohms, so how much interface do you expect the high impedance
(kilo-ohms) is capable into the balanced data line ?

[rbehm]

On 23.10.2014 12:41, Jasen Betts wrote:

> On 2014-10-23, Sylvia Else <syl...@not.at.this.address> wrote:
>> What to do with the USB cable shield at the device end?
>>
>> a) Leave it unconnected?
>>

>> b) Connect it directly to signal ground?
>>

>> c) Connect to signal ground via some impedance?
>

> Connect it to device shield. (in other words option a)
>

To add some real experience to all the theories posted in this thread:

I did DO-160 tests for EMC with a self developed system consisting of a
CPU system and a USB device. With the shield unconnected (one end as
well as both) we had a large, narrow peak at 120MHz, way above allowed
levels. Connecting both ends of the shield to the device case which was
also connected to signal ground on both sides there was no detectable
peak anymore.
The 120MHz came clearly from USB itself, some base frequency related to
the 480MBit/sec.

The argument about voltage differences if the device is powered be some
other supply do not make much sense to me. USB is not intended to be
routed over long distances, max cable length is 5m.
If there is such a big difference between the device cases/shields which
very often are also connected to signal ground (more less directly) USB
with its small signals will not work. You might even kill the USB ports.
A customer managed to do this. He used an ungrounded lab supply. When
unconnected the ground was at something like 120V-AC though capacitive
coupling in the transformer (240V-AC mains). Plugging the USB device
into the USB port of the test computer killed the port.

--
Reinhardt

---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

[k...@attt.bizz]

On Sun, 26 Oct 2014 00:46:21 -0500, Les Cargill

<lcarg...@comcast.com> wrote:

>k...@attt.bizz wrote:
>> On Sat, 25 Oct 2014 13:26:04 -0500, Les Cargill
>> <lcarg...@comcast.com> wrote:
><snip>
>>> USB is differential signals. So it's at least partially the same argument.
>>
>> No, it's not. The goals/priorities are completely different.
>
>Ground loops is ground loops.

No, they aren't. USB is not low-level analog, though there is the
issue with sound cards (hardly the reason USB exists). The needs are
very different.

>>> Thing about USB is that you're much less likely to have power from
>>> different service points. You are, however, likely to have wall warts,
>>> and wall warts emit noxious ground potentials sometimes.
>>
>>> Unfortunately, USB is usually cheap consumer stuff, so these things happen.
>>
>> The FCC still cares, though.
>>
>
>So an open ground on one end will cause violation of FCC? Might cause one?

A good possibility, sure. JL proposes that the grounds always be
connected at both ends *because* of FCC requirements. If he said
"usually", I'd agree. There are other needs, however. Engineering is
about tradeoffs.

>>> Had I never been bit by it, I wouldna said nothin'. :) In this case,
>>> I was able to sub in a different power supply.
>>
>> It? Which "it"?
>>
>
>Have a ground loop on USB cause bad performance.

You're lucky. I see problems all the time, particularly with laptops
and USB sound cards. It's one reason I got an isolated hub at work.

><snip>
>>> Transformer coupled audio is pretty rare these days.
>>
>> Wrong.
>>
>
>???

It's *very* common.

>Everybody uses that same 4-transistor circuit Mackie started in the
>'90s, outside of upmarket stuff. Just the transformer part cost alone
>for a Hammond 140NEX is $50ish bucks...

You're an idiot.

>I haven't seen anything new that was xformer-coupled in a long time.

You haven't dealt with *any* audio, then. You really should stick to
what you know.

[rickman]

On 10/26/2014 8:01 AM, rbehm wrote:
>
> The argument about voltage differences if the device is powered be some
> other supply do not make much sense to me. USB is not intended to be
> routed over long distances, max cable length is 5m.
> If there is such a big difference between the device cases/shields which
> very often are also connected to signal ground (more less directly) USB
> with its small signals will not work. You might even kill the USB ports.
> A customer managed to do this. He used an ungrounded lab supply. When
> unconnected the ground was at something like 120V-AC though capacitive
> coupling in the transformer (240V-AC mains). Plugging the USB device
> into the USB port of the test computer killed the port.

I'm not sure I understand. If the AC was reaching the equipment through
capacitive coupling in the transformer there should have been a pretty
high impedance to any appreciable current flow. Even the fine wire
inside the USB cable should have been able to conduct enough current to
equalize the grounds to within a small voltage.

I expect what you saw was some sort of true fault which allowed a much
higher current to flow and so a higher voltage to appear on the port.

--

Rick

[John Larkin]

On Sun, 26 Oct 2014 10:40:18 +0200, upsid...@downunder.com wrote:

>On Sat, 25 Oct 2014 10:59:28 -0700, John Larkin
><jla...@highlandtechnology.com> wrote:
>
>>>> I can't see why. That allows the unconnected shield to be a nice
>>>> antenna for hash or RF.
>>>>
>>>
>>>It's connected at one end. That should be enough to drain off EMI.
>>
>>A conductor grounded on one end is also known as a "vertical antenna."
>>It will resonate at multiple frequencies, and the voltage at the far
>>end will be jammed directly onto the wires inside the shield/antenna.
>
>A 5 m long quarter wave unipolar antenna will have the lowest
>resonance at 15 MHz as well as the 3/4 wave resonance at 45 MHz and
>5/4 resonance at 75 MHz. At these frequencies the antenna is resistive
>and capable of dumping some real power into the losses at the "load"
>grounded end.

There re multiple resonances at additional frequencies, well into the
cell phone sort of range. Not to mention the wideband effect, namely
treating the shield as an inductor/delay line, happy to twang whan hit
with fast spikes, like ESD discharges.

>
>At other frequencies, the antenna is more or less reactive and
>re-radiates more or less of the captured power immediately to the
>surrounding space, without flowing and dissipated into the load.
>
>Even with a resonant antenna, the impedance level along the antenna is
>not constant. The current is largest and voltage smallest at the load
>end (i.e. low impedance), while at the open end, the current is very
>low and voltage high i.e. high impedance of several kilo-ohms.
>
>The USB twisted pair transmission line has an impedance of about 100
>ohms, so how much interface do you expect the high impedance
>(kilo-ohms) is capable into the balanced data line ?

If there is RF voltage on the ungrounded end of the shield, it is
capacitively and inductively coupled into the wires inside. The
effective coupling impedance will be something less than 100 ohms. So
the RF voltage and ESD spikes get jammed into the electronics inside.

If you ground the shield, that doesn't happen.

[John Larkin]

EMI line filters do the same thing: couple half of the AC line voltage
into the device ground. The cable shield can short that out, but the
signal wires maybe can't.

[upsid...@downunder.com]

On Sun, 26 Oct 2014 08:52:38 -0700, John Larkin

<jla...@highlandtechnology.com> wrote:

>On Sun, 26 Oct 2014 10:40:18 +0200, upsid...@downunder.com wrote:
>
>>On Sat, 25 Oct 2014 10:59:28 -0700, John Larkin
>><jla...@highlandtechnology.com> wrote:
>>
>>>>> I can't see why. That allows the unconnected shield to be a nice
>>>>> antenna for hash or RF.
>>>>>
>>>>
>>>>It's connected at one end. That should be enough to drain off EMI.
>>>
>>>A conductor grounded on one end is also known as a "vertical antenna."
>>>It will resonate at multiple frequencies, and the voltage at the far
>>>end will be jammed directly onto the wires inside the shield/antenna.
>>
>>A 5 m long quarter wave unipolar antenna will have the lowest
>>resonance at 15 MHz as well as the 3/4 wave resonance at 45 MHz and
>>5/4 resonance at 75 MHz. At these frequencies the antenna is resistive
>>and capable of dumping some real power into the losses at the "load"
>>grounded end.
>
>There re multiple resonances at additional frequencies, well into the
>cell phone sort of range.

There are low impedance resonances at odd multiples of the quarter
wave resonance frequency and very high impedances at even multiples of
that resonances.

>Not to mention the wideband effect, namely
>treating the shield as an inductor/delay line, happy to twang whan hit
>with fast spikes, like ESD discharges.

What is wrong treating it as a transmission line ?

[Les Cargill]

k...@attt.bizz wrote:
> On Sun, 26 Oct 2014 00:46:21 -0500, Les Cargill
> <lcarg...@comcast.com> wrote:
>
>> k...@attt.bizz wrote:
>>> On Sat, 25 Oct 2014 13:26:04 -0500, Les Cargill
>>> <lcarg...@comcast.com> wrote:
>> <snip>
>>>> USB is differential signals. So it's at least partially the same argument.
>>>
>>> No, it's not. The goals/priorities are completely different.
>>
>> Ground loops is ground loops.
>
> No, they aren't. USB is not low-level analog, though there is the
> issue with sound cards (hardly the reason USB exists). The needs are
> very different.
>
>>>> Thing about USB is that you're much less likely to have power from
>>>> different service points. You are, however, likely to have wall warts,
>>>> and wall warts emit noxious ground potentials sometimes.
>>>
>>>> Unfortunately, USB is usually cheap consumer stuff, so these things happen.
>>>
>>> The FCC still cares, though.
>>>
>>
>> So an open ground on one end will cause violation of FCC? Might cause one?
>
> A good possibility, sure. JL proposes that the grounds always be
> connected at both ends *because* of FCC requirements. If he said
> "usually", I'd agree. There are other needs, however. Engineering is
> about tradeoffs.
>

Interesting.

>>>> Had I never been bit by it, I wouldna said nothin'. :) In this case,
>>>> I was able to sub in a different power supply.
>>>
>>> It? Which "it"?
>>>
>>
>> Have a ground loop on USB cause bad performance.
>
> You're lucky. I see problems all the time, particularly with laptops
> and USB sound cards. It's one reason I got an isolated hub at work.
>
>> <snip>
>>>> Transformer coupled audio is pretty rare these days.
>>>
>>> Wrong.
>>>
>>
>> ???
>
> It's *very* common.
>

I suspect it's less common than it used to be.

There's this whole range of prosumer stuff being used now. Some of it
is pretty good. Very strange...

>> Everybody uses that same 4-transistor circuit Mackie started in the
>> '90s, outside of upmarket stuff. Just the transformer part cost alone
>> for a Hammond 140NEX is $50ish bucks...
>
> You're an idiot.
>

We knew that.
And your mama dresses you funny.

People just have options without transformers now.

I'd a never figured this say thirty years ago, but ...

>> I haven't seen anything new that was xformer-coupled in a long time.
>
> You haven't dealt with *any* audio, then. You really should stick to
> what you know.
>

I have dealt with what I have dealt with. For the record, I have never
*designed* any.

I'd say it's closer to observer bias. The last nice console I used
( a D&R ) was transformerless; many are now. There's just a heck of
a lot more offerings now.

As proper solid state faux-balanced inputs became available, people
moved away from transformers.

I am pretty sure Neve and SSL use transformers still but those are sort
of legacy marques. Indeed; transformers sorta get sold now *as* a
"vintage" accoutrement.

--
Les Cargill

[Les Cargill]

rbehm wrote:
> On 23.10.2014 12:41, Jasen Betts wrote:
>> On 2014-10-23, Sylvia Else <syl...@not.at.this.address> wrote:
>>> What to do with the USB cable shield at the device end?
>>>
>>> a) Leave it unconnected?
>>>
>>> b) Connect it directly to signal ground?
>>>
>>> c) Connect to signal ground via some impedance?
>>
>> Connect it to device shield. (in other words option a)
>>
>
> To add some real experience to all the theories posted in this thread:
>
> I did DO-160 tests for EMC with a self developed system consisting of a
> CPU system and a USB device. With the shield unconnected (one end as
> well as both) we had a large, narrow peak at 120MHz, way above allowed
> levels. Connecting both ends of the shield to the device case which was
> also connected to signal ground on both sides there was no detectable
> peak anymore.
> The 120MHz came clearly from USB itself, some base frequency related to
> the 480MBit/sec.
>
> The argument about voltage differences if the device is powered be some
> other supply do not make much sense to me. USB is not intended to be
> routed over long distances, max cable length is 5m.

I understand.

So we had a broken wall-wart that put hash into the case ground of a
device. This caused performance problems on USB for this device.

If you put a scope on the ground of the device against the ground of
the PC it was to be connected to, you saw a lot of hash.

I presume case ground and signal ground were isolated.

> If there is such a big difference between the device cases/shields which
> very often are also connected to signal ground (more less directly) USB
> with its small signals will not work. You might even kill the USB ports.
> A customer managed to do this. He used an ungrounded lab supply. When
> unconnected the ground was at something like 120V-AC though capacitive
> coupling in the transformer (240V-AC mains). Plugging the USB device
> into the USB port of the test computer killed the port.
>

--

Les Cargill

[John Larkin]

Isolated warts are usually plastered with every UL/CSA/FCC/VDE sticker
imaginable. And most shoot huge amounts of switching spikes into their
outputs.

Consider a wart-powered USB hub. Would anyone suggest that the cable
from the PC to the hub have an unconnected shield at the hub end? And
unconnected shields at the ultimate device ends?

[k...@attt.bizz]

On Sun, 26 Oct 2014 11:55:22 -0500, Les Cargill

Less common, sure. Amplifier outputs aren't transformer coupled.
They're still *very* prevalent in professional audio.

>There's this whole range of prosumer stuff being used now. Some of it
>is pretty good. Very strange...

Don't know anything about "prosumer".

>>> Everybody uses that same 4-transistor circuit Mackie started in the
>>> '90s, outside of upmarket stuff. Just the transformer part cost alone
>>> for a Hammond 140NEX is $50ish bucks...
>>
>> You're an idiot.
>>
>
>We knew that.
>And your mama dresses you funny.

Ratty old clothes, too. ...since she's been dead for six years.

>People just have options without transformers now.

Sometimes.

>I'd a never figured this say thirty years ago, but ...

They'll never go away.

>>> I haven't seen anything new that was xformer-coupled in a long time.
>>
>> You haven't dealt with *any* audio, then. You really should stick to
>> what you know.
>>
>
>I have dealt with what I have dealt with. For the record, I have never
>*designed* any.
>
>I'd say it's closer to observer bias. The last nice console I used
>( a D&R ) was transformerless; many are now. There's just a heck of
>a lot more offerings now.
>
>As proper solid state faux-balanced inputs became available, people
>moved away from transformers.

For many uses, sure. Transformers still have many attributes that
silicon doesn't.

>I am pretty sure Neve and SSL use transformers still but those are sort
>of legacy marques. Indeed; transformers sorta get sold now *as* a
>"vintage" accoutrement.

Wrong again but you knew that.

[John Devereux]

Les Cargill <lcarg...@comcast.com> writes:

> Klaus Kragelund wrote:
>> USB data signals d+ and d- has CM range 0.8V to 2.5V and diff level
>> of 200mV, so it has some protection from ground loops, but not much.
>> Doesn't the USB standard define maximum cable length so a connection
>> to ground planes on each side is allowed since they assume small CM
>> voltages?
>>
>> Cheers
>>
>> Klaus
>>
>
> I would not assume signal ground is the same as case ground.

IME they always are in PCs for some reason (a good reason, presumably,
what with all those 100's of amps at multiple GHz).

> I've looked; I don't see any "ground lift" adapters for USB so it's
> maybe not something that comes up.

--

John Devereux

[rickman]

On 10/26/2014 2:35 PM, John Devereux wrote:
> Les Cargill <lcarg...@comcast.com> writes:
>
>> Klaus Kragelund wrote:
>>> USB data signals d+ and d- has CM range 0.8V to 2.5V and diff level
>>> of 200mV, so it has some protection from ground loops, but not much.
>>> Doesn't the USB standard define maximum cable length so a connection
>>> to ground planes on each side is allowed since they assume small CM
>>> voltages?
>>>
>>> Cheers
>>>
>>> Klaus
>>>
>>
>> I would not assume signal ground is the same as case ground.
>
> IME they always are in PCs for some reason (a good reason, presumably,
> what with all those 100's of amps at multiple GHz).

They are only the same at DC which is the whole point of the concern
with ground loops.

--

Rick

[rickman]

On 10/26/2014 12:55 PM, Les Cargill wrote:
>
> As proper solid state faux-balanced inputs became available, people
> moved away from transformers.

Faux-balanced inputs? What exactly is that? If you mean they are
balanced, but not floating, ok.

--

Rick

[Les Cargill]

k...@attt.bizz wrote:
> On Sun, 26 Oct 2014 11:55:22 -0500, Les Cargill
> <lcarg...@comcast.com> wrote:
>

<snip>

>> I suspect it's less common than it used to be.
>
> Less common, sure. Amplifier outputs aren't transformer coupled.
> They're still *very* prevalent in professional audio.
>

It... depends. It's gotten all very strange over the last few years.

>> There's this whole range of prosumer stuff being used now. Some of it
>> is pretty good. Very strange...
>
> Don't know anything about "prosumer".
>

It's basically higher-performing MI store stuff.

I have one sitting here to my left; noise is -102dB and THD+N is like
unto 0.001%, but it's low enough that I don't trust the cables I use...

It's not capital equipment. $500.

>>>> Everybody uses that same 4-transistor circuit Mackie started in the
>>>> '90s, outside of upmarket stuff. Just the transformer part cost alone
>>>> for a Hammond 140NEX is $50ish bucks...
>>>
>>> You're an idiot.
>>>
>>
>> We knew that.
>> And your mama dresses you funny.
>
> Ratty old clothes, too. ...since she's been dead for six years.
>

My apologies; I had no way of knowing.

>> People just have options without transformers now.
>
> Sometimes.
>
>> I'd a never figured this say thirty years ago, but ...
>
> They'll never go away.
>

Agreed.

>>>> I haven't seen anything new that was xformer-coupled in a long time.
>>>
>>> You haven't dealt with *any* audio, then. You really should stick to
>>> what you know.
>>>
>>
>> I have dealt with what I have dealt with. For the record, I have never
>> *designed* any.
>>
>> I'd say it's closer to observer bias. The last nice console I used
>> ( a D&R ) was transformerless; many are now. There's just a heck of
>> a lot more offerings now.
>>
>> As proper solid state faux-balanced inputs became available, people
>> moved away from transformers.
>
> For many uses, sure. Transformers still have many attributes that
> silicon doesn't.
>

Of course they do. There's nothing better for iso and they impart a
sound.

That's... kinda how we got started here to begin with.

>> I am pretty sure Neve and SSL use transformers still but those are sort
>> of legacy marques. Indeed; transformers sorta get sold now *as* a
>> "vintage" accoutrement.
>
> Wrong again but you knew that.
>

I mean something like this:
http://www.manley.com/mcore.php

It's all about packaging options these days.

--
Les Cargill

[Les Cargill]

Basically.

--
Les Cargill

[Maynard A. Philbrook Jr.]

In article <dvbq4a1rq3vr8ctj6...@4ax.com>,
jla...@highlandtechnology.com says...

> >I presume case ground and signal ground were isolated.
> >
> >> If there is such a big difference between the device cases/shields which
> >> very often are also connected to signal ground (more less directly) USB
> >> with its small signals will not work. You might even kill the USB ports.
> >> A customer managed to do this. He used an ungrounded lab supply. When
> >> unconnected the ground was at something like 120V-AC though capacitive
> >> coupling in the transformer (240V-AC mains). Plugging the USB device
> >> into the USB port of the test computer killed the port.
> >>
>
> Isolated warts are usually plastered with every UL/CSA/FCC/VDE sticker
> imaginable. And most shoot huge amounts of switching spikes into their
> outputs.

If that was the case with the wart unloaded, it should be removed off
the store shelves. Only hardwired supplies should do that unloaded out
of circuit and not in circuit. But as you say, it's a wall wart, so it
should not be doing that.

> Consider a wart-powered USB hub. Would anyone suggest that the cable
> from the PC to the hub have an unconnected shield at the hub end? And
> unconnected shields at the ultimate device ends?

Well, most hubs I've seen don't do much in the way of a grounding the
shield inside, but more to your point, the USB cables are generally
shield connected on both ends anyway, that is, if you have that type of
cable. I have a couple over here that has no shields at all, just
twisted pairs.

If the manufacture of the product being connected to the end of the
cable sees a problem with DC ground loops, which can happen of course,
but still needs an RF shield, they usually capacitor couple the shield
to the device ground and also may include a series R to drop Q and
introduce some resistance in a high RF area.

A DC ground loop via the shield can damage the front end of sensitive
devices.
When unexpected currents appear in between chassis, currents
can pass through the shields, enough so that the magnetic coupling can
pulse this current into the inputs and outputs of the devices on each
end of the cable. In the industrial world, we always connect shield
on one end of a data line and bring the shield close to the end of the
leads at the other end, taped off.

I have always past comments to the guys I work with about checking the
path of the SG (signal Ground) connection to device to true ground and
if there does not seem to be a DC path, then simply connect the shield
to the device and the other end. The problem is that a lot of devices
out there do not supply a separate SG connection point, even if they do
simply tie directly to chassis. When it's chassis ground only being
supplied, we make sure one end isn't DC connected.

Have you ever tried putting a scope on the inside conductor of a coax
and then pulsed some current through the shield?

Btw.
I use shielded grounds in my shop for my computer, radios, amps etc,
otherwise, I'd have so much RF noise I wouldn't be able to use any
of my RF gear, due mostly to the computers.

I am sure you know all of this and I most likely wasted my time. ;)

Jamie

[Les Cargill]

Yep.

> Consider a wart-powered USB hub. Would anyone suggest that the cable
> from the PC to the hub have an unconnected shield at the hub end? And
> unconnected shields at the ultimate device ends?
>

Well put. Probably not. This is where isolation as krw mentioned would
be the thing to do.

--
Les Cargill

[John Larkin]

On Sun, 26 Oct 2014 18:35:52 +0000, John Devereux
<jo...@devereux.me.uk> wrote:

>Les Cargill <lcarg...@comcast.com> writes:
>
>> Klaus Kragelund wrote:
>>> USB data signals d+ and d- has CM range 0.8V to 2.5V and diff level
>>> of 200mV, so it has some protection from ground loops, but not much.
>>> Doesn't the USB standard define maximum cable length so a connection
>>> to ground planes on each side is allowed since they assume small CM
>>> voltages?
>>>
>>> Cheers
>>>
>>> Klaus
>>>
>>
>> I would not assume signal ground is the same as case ground.
>
>IME they always are in PCs for some reason (a good reason, presumably,
>what with all those 100's of amps at multiple GHz).
>

Don't PC motherboards have multiple grounded mounting holes?

Single-point grounding would be an EMI nightmare.

When in doubt, ground it out.

[Maynard A. Philbrook Jr.]

In article <a0nq4a5bv38gu1s1q...@4ax.com>,
jla...@highlandtechnology.com says...

> >>> Klaus
> >>>
> >>
> >> I would not assume signal ground is the same as case ground.
> >
> >IME they always are in PCs for some reason (a good reason, presumably,
> >what with all those 100's of amps at multiple GHz).
> >
>
> Don't PC motherboards have multiple grounded mounting holes?
>
> Single-point grounding would be an EMI nightmare.
>
> When in doubt, ground it out.
>

We have a slogan similar to that at work..

"When in doubt, ship it out"

Then, for the longest time there was a banner hanging in the
shipping area, quote "The customer is the next inspector", someone
got smart and decided to add a little at the beginning for all to see,
"Don't worry, The customer is the next inspector", that actually was
up there for a while before those that mattered noticed it.

That area of the operation has a lot of QC testing taking place just
prior to package the product for shipment.

I am sure you have QC check points very close to the shipping process,
too.

Jamie

[k...@attt.bizz]

On Sun, 26 Oct 2014 14:18:28 -0500, Les Cargill

<lcarg...@comcast.com> wrote:

>k...@attt.bizz wrote:
>> On Sun, 26 Oct 2014 11:55:22 -0500, Les Cargill
>> <lcarg...@comcast.com> wrote:
>>
><snip>
>>> I suspect it's less common than it used to be.
>>
>> Less common, sure. Amplifier outputs aren't transformer coupled.
>> They're still *very* prevalent in professional audio.
>>
>
>It... depends. It's gotten all very strange over the last few years.

I was discounting audiophoolery. Infinitesimal market.

>>> There's this whole range of prosumer stuff being used now. Some of it
>>> is pretty good. Very strange...
>>
>> Don't know anything about "prosumer".
>>
>
>It's basically higher-performing MI store stuff.

I know what it is. Never looked inside.

>I have one sitting here to my left; noise is -102dB and THD+N is like
>unto 0.001%, but it's low enough that I don't trust the cables I use...

Do you trust the specs? Do you really care?

>It's not capital equipment. $500.
>
>>>>> Everybody uses that same 4-transistor circuit Mackie started in the
>>>>> '90s, outside of upmarket stuff. Just the transformer part cost alone
>>>>> for a Hammond 140NEX is $50ish bucks...
>>>>
>>>> You're an idiot.
>>>>
>>>
>>> We knew that.
>>> And your mama dresses you funny.
>>
>> Ratty old clothes, too. ...since she's been dead for six years.
>>
>
>My apologies; I had no way of knowing.

I took no offense but as long as you were going there...

>>> People just have options without transformers now.
>>
>> Sometimes.
>>
>>> I'd a never figured this say thirty years ago, but ...
>>
>> They'll never go away.
>>
>
>Agreed.
>
>>>>> I haven't seen anything new that was xformer-coupled in a long time.
>>>>
>>>> You haven't dealt with *any* audio, then. You really should stick to
>>>> what you know.
>>>>
>>>
>>> I have dealt with what I have dealt with. For the record, I have never
>>> *designed* any.
>>>
>>> I'd say it's closer to observer bias. The last nice console I used
>>> ( a D&R ) was transformerless; many are now. There's just a heck of
>>> a lot more offerings now.
>>>
>>> As proper solid state faux-balanced inputs became available, people
>>> moved away from transformers.
>>
>> For many uses, sure. Transformers still have many attributes that
>> silicon doesn't.
>>
>
>Of course they do. There's nothing better for iso and they impart a
>sound.

They shouldn't. That's the point.

>That's... kinda how we got started here to begin with.

No, their purpose is *not* to "impart a sound". That's why they're
used. Isolation - yes. Balancing - yes. Altering the sound? Nope.
If you want that, use toobz. ;-)

>>> I am pretty sure Neve and SSL use transformers still but those are sort
>>> of legacy marques. Indeed; transformers sorta get sold now *as* a
>>> "vintage" accoutrement.
>>
>> Wrong again but you knew that.
>>
>
>I mean something like this:
>http://www.manley.com/mcore.php
>
>It's all about packaging options these days.

Well, for certain values of "it", I suppose.

[John Larkin]

On Sun, 26 Oct 2014 17:21:27 -0500, "Maynard A. Philbrook Jr."
<jamie_...@charter.net> wrote:

>In article <a0nq4a5bv38gu1s1q...@4ax.com>,
>jla...@highlandtechnology.com says...
>> >>> Klaus
>> >>>
>> >>
>> >> I would not assume signal ground is the same as case ground.
>> >
>> >IME they always are in PCs for some reason (a good reason, presumably,
>> >what with all those 100's of amps at multiple GHz).
>> >
>>
>> Don't PC motherboards have multiple grounded mounting holes?
>>
>> Single-point grounding would be an EMI nightmare.
>>
>> When in doubt, ground it out.
>>
>
> We have a slogan similar to that at work..
>
> "When in doubt, ship it out"

Funny. When we have any doubt about something, we don't ship.

>
> Then, for the longest time there was a banner hanging in the
>shipping area, quote "The customer is the next inspector", someone
>got smart and decided to add a little at the beginning for all to see,
>"Don't worry, The customer is the next inspector", that actually was
>up there for a while before those that mattered noticed it.
>
> That area of the operation has a lot of QC testing taking place just
>prior to package the product for shipment.
>
> I am sure you have QC check points very close to the shipping process,
>too.
>
>Jamie

[k...@attt.bizz]

On Sun, 26 Oct 2014 15:30:58 -0700, John Larkin
<jla...@highlandtechnology.com> wrote:

>On Sun, 26 Oct 2014 17:21:27 -0500, "Maynard A. Philbrook Jr."
><jamie_...@charter.net> wrote:
>
>>In article <a0nq4a5bv38gu1s1q...@4ax.com>,
>>jla...@highlandtechnology.com says...
>>> >>> Klaus
>>> >>>
>>> >>
>>> >> I would not assume signal ground is the same as case ground.
>>> >
>>> >IME they always are in PCs for some reason (a good reason, presumably,
>>> >what with all those 100's of amps at multiple GHz).
>>> >
>>>
>>> Don't PC motherboards have multiple grounded mounting holes?
>>>
>>> Single-point grounding would be an EMI nightmare.
>>>
>>> When in doubt, ground it out.
>>>
>>
>> We have a slogan similar to that at work..
>>
>> "When in doubt, ship it out"
>
>Funny. When we have any doubt about something, we don't ship.

Agreed. Getting it back is always far more expensive than fixing it
in the first place.

[Reinhardt Behm]

Les Cargill wrote:

>> To add some real experience to all the theories posted in this thread:
>>
>> I did DO-160 tests for EMC with a self developed system consisting of a
>> CPU system and a USB device. With the shield unconnected (one end as
>> well as both) we had a large, narrow peak at 120MHz, way above allowed
>> levels. Connecting both ends of the shield to the device case which was
>> also connected to signal ground on both sides there was no detectable
>> peak anymore.
>> The 120MHz came clearly from USB itself, some base frequency related to
>> the 480MBit/sec.
>>
>> The argument about voltage differences if the device is powered be some
>> other supply do not make much sense to me. USB is not intended to be
>> routed over long distances, max cable length is 5m.
>
> I understand.
>
> So we had a broken wall-wart that put hash into the case ground of a
> device. This caused performance problems on USB for this device.
>
> If you put a scope on the ground of the device against the ground of
> the PC it was to be connected to, you saw a lot of hash.
>
> I presume case ground and signal ground were isolated.
>

I guess you did not understand correctly.
There was no wall wart. DO-160 testing is for aviation, you would not want
to use something like this in aircraft.
The case ground and signal ground where connected. The power for the USB
device came from the same source as for the USB host.
There where also no performance problems. The problem was radiated emission
which was way above limits when the shield was not connected at both sides
to the case. With connection at both sides the emission was bare measurable.
--
Reinhardt

[whit3rd]

On Saturday, October 25, 2014 3:50:46 PM UTC-7, Phil Allison wrote:
> k...@attt.bizz wrote:

> > It's common to
> > unground the shield on RCA cables, too (particularly at low-level
> > sources, like turn tables).
>
>
> ** Huh ?? Wot utter nonsense.
>
> RCA leads supply the *only* ground connection to most audio items that use them - cos for the last 30 years or more almost every thing made is Class II or "double insulated" so does not have a safety ground connection.

Halftrue, not nonsense. It's common practice to connect the left channel shield to
chassis ground, and leave the right channel shield floating. It's easy to see this,
on a turntable, because this connection is made at the phono cartridge.

[Phil Allison]

** Even worse, irrelevant nonsense.

FYI:

Phono pickups are *floating* sources - so cable shields can only possibly be grounded at the amplifier end.

Merely plugging the RCA leads in does this and that is mighty hard to avoid.

Leaving off one of the ground connections to the two coils in the PU normally means loss of signal from that channel.



.... Phil

[whit3rd]

On Sunday, October 26, 2014 8:31:29 PM UTC-7, Phil Allison wrote:
> whit3rd wrote:
>
...

> > Halftrue, not nonsense. It's common practice to connect the left channel shield to
> > chassis ground, and leave the right channel shield floating. It's easy to see this,
> > on a turntable, because this connection is made at the phono cartridge.
>
>
> ** Even worse, irrelevant nonsense.
>
> FYI:
>
> Phono pickups are *floating* sources - so cable shields can only possibly be grounded at the amplifier end.
>
> Merely plugging the RCA leads in does this and that is mighty hard to avoid.

Nor is it avoided. I never said the shield was unconnected at BOTH ends. Only
that the right channel shield does not connect to any chassis or shield inside the
phonograph.

>
> Leaving off one of the ground connections to the two coils in the PU normally means loss of signal from that channel.

Not leaving off the ground connection to the coil. Only, not connecting the second
(in this case right channel ground) cable shield to chassis or shield of the phonograph.
Of COURSE you connect it to the phono cartridge right ground pin, but NOT to
the phono cartridge case (which is grounded through the left channel ground).

[josephkk]

On Sat, 25 Oct 2014 12:50:52 +0300, upsid...@downunder.com wrote:

>On Fri, 24 Oct 2014 17:41:43 -0700 (PDT), Phil Allison
><palli...@gmail.com> wrote:
>
>>upsid...@downunder.com wrote:
>>
>>>
>>> This applies both to 120/240 V antiphase feeds in the US as well as to
>>> three phase 230/400 V systems in the rest of the world.
>>>
>>> The principal idea in both systems is that with _balanced_ loads, the
>>> neutral wire current cancels and the neutral wire potential is the
>>> same as the ground electrode potential all over the place.
>>
>> ** No one who is not insane ever assumes that nonsense to be the case.
>>
>>Neutral current is assumed to be either the same or somewhat more than the maximum current in one phase. Max voltage drops along cables can be computed from this.
>>
>>
>>> Assume an apartment building with two stairs with separate feeders
>>> along the stairs. There are going to be unequal voltage drops in the
>>> phase as well as neutral conductor at both stairs.
>>>
>>> At some higher floor, measure the mains neutral/ground connector
>>> potential against the potential of the same floor of the adjacent
>>> stairs. With absolutely equal and symmetrical load, the neutral/ground
>>> potential would be equal.
>>>
>>> In practice, this is not the case. Connect an unisolated RS-232 or
>>> USB cable between the stairs and the shields might quite well be 1-5
>>> Vrms at different potential.
>>
>>** What halluncinary drugs are you on today ??
>>
>> No-one connects signal grounds to the damn NEUTRAL !!!
>>
>> Doing that would be CRIMINAL !!
>>
>>
>>>
>>> Guess what is going to happen if you connect the shields together ?
>>
>>
>> ** Nothing - cos the earth conductor carries virtually no current and so has no voltage drops.
>>
>>
>>> With a bad unbalance load between the stairs feeders, a huge amount
>>> (several amperes) may flow from staircase 1 to staircase 2 neutral
>>> through your signal cable shield, causing _a_lot_ havoc to your
>>> communication.
>>
>>
>>** What halluncinary drugs are you on today ??
>>
>> No-one connects signal grounds to the damn NEUTRAL !!!
>
>Not directly of course, but look at the big picture.
>
>Take a look at en.wikipedia.org/wiki/Earthing_system and especially
>TN-C-S
>
>Do you expect that the Neutral (N) and protective ground (PE) wires
>would run the whole way back to the power plant. That would be a true
>TN-S system with total of 5 wires. In practice the distribution system
>is some form of TN-C-S, in which the PEN (combined protective earth
>and neutral starts at the distribution transformer star point. This
>PEN conductor is split into PE and N at some point towards the
>customer.
>
>This splitting point could be at the main distribution panel, in any
>sub distribution panel or in apartment specific distribution panel. It
>would not at all be strange to have a common PEN conductor in the
>raiser cables in two separate stairs and hence different ground
>potential between stairs.
>
>Some older regulation allow the PEN to PE and N in each mains socket.
>Thus, the equipment mains cord, is the only -S part in TN-C-S
>
>If the equipment signal ground or shield are connected to the
>equipment local PE ground, there may be ground potential differences
>between devices even in the same room, same apartment and at least
>between buildings.
>
>>
>> Doing that would be CRIMINAL !!
>>
>>
>>>
>>> For this reason, one should avoid connecting cable shields together.
>>>
>> ** More insane crap.
>>
>> You need kicking off this NG pal.
>>
>>
>>> >When you connect a signal generator to an oscilloscope with a BNC-BNC
>>> >coax cable, do you strip out the shield on the scope end of the cable?
>>>
>>> As long as you are using a single mains outlet to feed a grounded
>>> extension cable for these two devices, the device ground potentials
>>> should be within a few millivolts.
>>
>> ** Same goes for any AC power outlet in the same premises.
>>
>>Neutral current is not involved unless there is a dangerous fault in the AC supply wiring.
>>
>>
>>.... Phil
>>
>>

Once again Phil Allison gets outside his demesnes and speaks incorrectly
again. Phase imbalances are really quite frequent and the reason that
neutral conductors are the same size as phase conductors today in the NEC
and related standards and regulations.

?-)

[Phil Allison]

whit3rd wrote:

> Phil Allison wrote:
> > whit3rd wrote:
> >
> ...
> > > Halftrue, not nonsense. It's common practice to connect the left channel shield to
> > > chassis ground, and leave the right channel shield floating. It's easy to see this,
> > > on a turntable, because this connection is made at the phono cartridge.
> >
> >
> > ** Even worse, irrelevant nonsense.
> >
> > FYI:
> >
> > Phono pickups are *floating* sources - so cable shields can only possibly be grounded at the amplifier end.
> >
> > Merely plugging the RCA leads in does this and that is mighty hard to avoid.
>
> Nor is it avoided.

** But YOU said it was - wanker.

> I never said the shield was unconnected at BOTH ends.

** That is exactly what YOU posted:

" ... and leave the right channel shield floating ".

BTW: Better learn what the word "floating" means sometime.

> > Leaving off one of the ground connections to the two coils in the PU normally means loss of signal from that channel.
>

> Not leaving off the ground connection to the coil. Only, not connecting the second
> (in this case right channel ground) cable shield to chassis or shield of the phonograph.
> Of COURSE you connect it to the phono cartridge right ground pin, but NOT to
> the phono cartridge case (which is grounded through the left channel ground).

** What a load of incomprehensible bollocks that could have been stated in one short sentence:

" The metal case of a phono PU is generally connected to one RCA lead shield."

The other common way is that it connects to an all metal tone arm that has its own ground wire.

NONE of which is even on-topic.


... Phil

[Reinhardt Behm]

rickman wrote:

> On 10/26/2014 8:01 AM, rbehm wrote:
>>
>> The argument about voltage differences if the device is powered be some
>> other supply do not make much sense to me. USB is not intended to be
>> routed over long distances, max cable length is 5m.

>> If there is such a big difference between the device cases/shields which
>> very often are also connected to signal ground (more less directly) USB
>> with its small signals will not work. You might even kill the USB ports.
>> A customer managed to do this. He used an ungrounded lab supply. When
>> unconnected the ground was at something like 120V-AC though capacitive
>> coupling in the transformer (240V-AC mains). Plugging the USB device
>> into the USB port of the test computer killed the port.
>

> I'm not sure I understand. If the AC was reaching the equipment through
> capacitive coupling in the transformer there should have been a pretty
> high impedance to any appreciable current flow. Even the fine wire
> inside the USB cable should have been able to conduct enough current to
> equalize the grounds to within a small voltage.
>
> I expect what you saw was some sort of true fault which allowed a much
> higher current to flow and so a higher voltage to appear on the port.
>
The problem was, they first connected the USB device to the ungrounded power
supply. Since there was no connection to ground the capacitively coupled AC
was on the signal ground.
When thy connected the device (with unconnected shield) to the PC these AC
voltage hit the USB inputs and pfft..
The input impedance of the USB signal lines was to high to gt the coupled
voltage down to acceptable levels. It happened only sometimes depending what
pin got the connection first. When GND was first it shorted the AC to ground
and nothing happened.

This does not have much to do with USB itself. It is a common problem that
any ungrounded AC powered supply creates. I had the same effect with a HP
printer in the days of centronics printer connectors. The printer had an
external PSU without a PE pin in its mains plug. Fortunately I first touched
the connector shield at the PC printer port with the cable connector and saw
the sparks.

The USB connector is designed in a way that normally the connector frame
(shield) first touches. This protects from stray voltages as well as ESD
_iff_ the shield is connected on both sides of the cable.

--
Reinhardt

[Reinhardt Behm]

John Larkin wrote:

> On Fri, 24 Oct 2014 22:59:44 -0500, Les Cargill
> <lcarg...@comcast.com> wrote:
>
>>John Larkin wrote:
>>> On Thu, 23 Oct 2014 07:27:48 +0300, upsid...@downunder.com wrote:
>>>
>>>> On Thu, 23 Oct 2014 14:32:37 +1100, Sylvia Else

>>>> <syl...@not.at.this.address> wrote:
>>>>
>>>>> What to do with the USB cable shield at the device end?
>>>>>
>>>>> a) Leave it unconnected?
>>>>

>>>> This is good against electrostatically connected interference.

>>>>
>>>>> b) Connect it directly to signal ground?
>>>>

>>>> This is a bad idea, if the cable ends are connected to different
>>>> grounds, creating different ground potentials and hence large currents
>>>> to equalize the potentials. The current is often at mains frequency
>>>> and harmonics.
>>>
>>> We *want* to equalize those potentials! The shield helps do that.

>>>
>>> When you connect a signal generator to an oscilloscope with a BNC-BNC
>>> coax cable, do you strip out the shield on the scope end of the cable?
>>>
>>>
>>
>>

>>With the BNC, SFAIK that's a single-ended connection - the lead is in
>>the middle, the coax is the return.
>>
>>With an XLR, you may want to float ( open ) one end of the shield for a
>>balanced connection. There may be a case ground potential difference
>>from box 1 to box 2.

>
> I can't see why. That allows the unconnected shield to be a nice
> antenna for hash or RF.
>

> For minimum common-mode on the differential signal, it's best to
> connect the shield at both ends. That reduces the noted potential
> difference.
>
>
>>
>>The only question is which model works best for USB.
>>
>>http://www.usbpinout.net/
>>
>>I see a D+/D- pair. And a ground. Looks balanced to me. You may want
>>the shield connected only at one end.
>
> In that case, disconnect the ground wire too.
>
And give the full difference in ground potential onto your signal lines.
They like it if they are sometimes fed it bit more voltage than usual :-)
--
Reinhardt

[Reinhardt Behm]

Michael A. Terrell wrote:

>
> John Larkin wrote:
>>
>> Even transformer coupled audio signals have some finite common-mode
>> rejection at the receive end. An open shield is an antenna that jams
>> cm voltages onto all the wires inside the cable.
>
>
> You connect the shield at one end. The input. Otherwise you can have
> all kinds of problems.
>

USB is bi-directional. So which side is the input?

--
Reinhardt

[upsid...@downunder.com]

On Sun, 26 Oct 2014 17:21:27 -0500, "Maynard A. Philbrook Jr."
<jamie_...@charter.net> wrote:

>In article <a0nq4a5bv38gu1s1q...@4ax.com>,
>jla...@highlandtechnology.com says...
>> >>> Klaus
>> >>>
>> >>
>> >> I would not assume signal ground is the same as case ground.
>> >
>> >IME they always are in PCs for some reason (a good reason, presumably,
>> >what with all those 100's of amps at multiple GHz).
>> >
>>
>> Don't PC motherboards have multiple grounded mounting holes?
>>
>> Single-point grounding would be an EMI nightmare.
>>
>> When in doubt, ground it out.
>>
>
> We have a slogan similar to that at work..
>
> "When in doubt, ship it out"
>
> Then, for the longest time there was a banner hanging in the
>shipping area, quote "The customer is the next inspector", someone
>got smart and decided to add a little at the beginning for all to see,
>"Don't worry, The customer is the next inspector", that actually was
>up there for a while before those that mattered noticed it.

That would be "customer-oriented testing" :-)

In many cases, this has been the standard method in software industry
since mainframes OSs.

[upsid...@downunder.com]

It seems that some Chinese manufacturer are satisfied with 95 % yield.

If the customer complains about a defective unit, just send a new
device (hoping that the replacement unit is not defective), without
even asking the customer to return the defective unit.

[David Eather]

Long time since I did anything that needed any knowledge in this area, but
what Phil is saying seems to be about right - at least in OZ.

[rickman]

I recall a manager attempting to instill a sense of duty in us by
explaining how a Sony TV is turned on for the first time by the
customer. Their defect rate is so low that they can do this it would
seem.

--

Rick

[Michael A. Terrell]

Tim Williams wrote:
>
> "Michael A. Terrell" <mike.t...@earthlink.net> wrote in message
> news:XuOdnYLK9YUXt9HJ...@earthlink.com...
> >> I can't believe how much misinformation there is, still persisting
> >> today.
> >
> >
> > Who are you talking about? I've had to cut hundreds of connections
> > to eliminate ground loop problems in studios. Everything run on a single
> > phase, of 120/208 three phase. This was all balanced, transformer
> > coupled audio lines with a transformer at each end and inside metal
> > equipment cases.
>
> Try that with an EMC susceptibility test...


Try to test it in the near field of a 50 KW AM radio transmitter.


> If it was humming even with isolation (and the shield made any difference
> despite the supposed isolation), that's their problem. Hey, just because
> it costs $10k doesn't mean it was made at all correctly...
>
> ...And it doesn't mean that cutting the shield is ever a good idea in
> general. Maybe it works in one special case, maybe it works in a few. It
> ain't a general rule, and there's good reason, whether you understand it
> or not.


You have a lot to learn, child.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

[Michael A. Terrell]

Read what I replied to. AUDIO. Not USB.