An RS232 cable question or two

[et...@whidbey.com]

Greetings RS232 Gurus,
I have a couple machine tools that connect to a computer through
RS232. I now need to run a longer cable for one of the machines.
Looking online it seems like the biggest limiting factor is the
capacitance of the cable. I have a reel of cable with three twisted
pairs and no shield that's about 300 feet long. Measuring the
capacitance with my multimeter I find the capacitance is less than
2300pf. This is fine as near as I can tell because I only need to run
about 60 feet and the baud rate I need is 19600. I don't know how
accurate the meter is except for what the documentation says and it
seems like the measurement is accurate enough. So I think the baud
rate I want to use will be OK. But I have two questions. First, I
measured the capacitance of each pair of wires by stripping the ends
and connecting the meter to these ends without touching with my
fingers. Each pair was pretty close to the same measurement. Is this
the proper way to test the capacitance of the cable? Second, there is
no shield in the cable. Since I only need one pair of the three pairs
in the cable can I just use the four remaining wires connected
together at one end as a shield since all the wires are twisted
together? Or should I just go out and buy a cable with a shield and
put this reel back into storage with all my other wire?
Thanks,
Eric

[Jon Kirwan]

I'm just a hobbyist, not an expert. Others can and should
chip in. But I have the RS-232 spec handy and figured I'd
give it a shot.

I can't answer about how good your capacitance measurement
is. Probably should be done with an LCR meter set to 10kHz
and 100kHz. Good twisted pair runs 15pF/ft or less... but
cable for RS-232 would be roughly 40pF-50pF/ft. Your 300' of
cable measurement comes out at 7.7pF/ft. Which doesn't seem
likely.

RS-232D specifies at most 2500pF for data rates up to 9600
and receiver impedance between 3k and 7k Ohm. The bit times
for 9600 is 104us. So the tau=RC=7k*2500pF=17.5us. That's 6
tau per bit time. At 19200, the bit time is 52us, so the tau
should be half of 17.5us or 8.75us and the capacitance should
be 1250pF or less. If the cable were older RS-232 cable at
50pF/ft, that would limit you to 25'. But since you are using
unshielded twisted pair, it's more likely to be closer to the
15pF/ft figure. Which gets you three times as far, or maybe
75' or so.

Also, actual testing of cables shows that the RS-232 driver
and receiver pairs actually do better than the standard says.
Receivers usually aren't 7k Ohm, for example, but less. So
that's good.

Since these were based on worst case 7k ohm receivers... just
guessing at this... I'd say you've got a good shot at it
working okay at 60' and 19200. Of course, none of this takes
into account noise and nearby machinery effects. Or ground
loop currents, if any. But I'd say it's worth a shot and that
you will probably be okay. (I know I've run more than 60' of
non-twisted pair cable at 19200 without any trouble --- from
a VAX 11/780 computer to consoles placed outside the raised
floor, air-conditioned room.)

But maybe an expert will chip in and set me straight.

Jon

[Jim Thompson]

You could build your own driver with "pre-comp"... boost the leading
edge dramatically. I've done that for ~1000' of cable. That's so old
a design (pre-CAD) it's own paper (pre-CAD). I'll see if I can find
it in my archives.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| 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.

[Les Cargill]

Really, obtain a couple of 422 or 485 to 232 converters and do
that. Even if it works while you're looking at it*, the additional
CMRR of 485/422 is vastly worth it, and the adapters don't have
to cost much.

*the gremlins always come out when you are looking somewhere else.

<http://www.serialcomm.com/serial_rs232_converters/rs232_to_rs485_converters/rs232_to_rs485_converter/rs232_to_rs485.product_general_info.aspx>

--
Les Cargill

[et...@whidbey.com]

On Wed, 19 Dec 2012 22:42:47 -0800, Jon Kirwan
<jo...@infinitefactors.org> wrote:

Thanks for the reply Jon. I was dubious about how accurately my meter
was measuring the capacitance. But since the cable is pretty new, and
I'm running less than 75 feet, I think I'll be OK. I do have the
option of slowing down the data rate and programs now transfer between
machines in less than 30 seconds usually so slowing the data rate down
won't really slow me down much. Especially since I'm only transferring
programs about once a day.
Thanks,
Eric

[petrus bitbyter]

<et...@whidbey.com> schreef in bericht
news:igt4d8l9vc69e32sv...@4ax.com...

Cables - all cables - have resistance, conductance, capacitance and
inductance. Measuring the capacitance of long cables will be influenced by
the inductance. The rate of that influence depends on the cable, its length
and the measuring method. To get an indication of the capacitance you'd
better take a shorter piece of cable. Let's say a 5ft and a 10ft piece.
Inductance will have hardly influence as capacitance is (almost) always
measured at relative low frequencies. If a double length measures double the
capacitance you know you're on the right track.

Whether or not an unshielded cable will do, depends mainly on the disturbing
EM-fields in the environment. Engines, switchers, triac controls and the
like. I've used longer cat 5 cable in an office environment without
problems. But I also had to provide a solution for the same length of
cables - though at higher speed - in a factory hall. So if it's no problem
to try, give it a try. Otherwise you'd better go for the RS232 to RS458
converters. (If even they fail, you can go for glassfibre. I once had to do
for a very long cable.)

petrus bitbyter

[John Larkin]

On Wed, 19 Dec 2012 18:36:28 -0800, et...@whidbey.com wrote:

Try it! I've run RS232 over a city block at 9600.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

[petrus bitbyter]

"John Larkin" <jjla...@highNOTlandTHIStechnologyPART.com> schreef in
bericht news:k5bad8h4kn39ovtla...@4ax.com...

I've done similar without any problem.

But I also had some customer leading a cable along a gutter for maybe 25m
happily paying the bill 4 or 5 times a year when his equipment was blown and
had to be replaced. Relative cheap office equipment but nevertheless. He
kept saying it was only temporarily and he was right off course. Everything
in this life is temporarily isn't it? Though this time it took over 2 years
if memory serves.

On another location a double shielded ground cable of about 100m did 9600
RS458 very well until the next thunderstorm. Eventually I used glass fiber
to solve the problem.

petrus bitbyter

[whit3rd]

On Wednesday, December 19, 2012 6:36:28 PM UTC-8, et...@whidbey.com wrote:

> I have a couple machine tools that connect to a computer through
> RS232. I now need to run a longer cable for one of the machines.

[60 feet at 19+ kbaud]

In a machine-shop environment, it might be better to use a differential
protocol for long cables. One could consider Ethernet twisted-pair cable, and
a little Ethernet/serial converter near the machine tool. It's probably going to
be useful to have Ethernet wires later, anyhow.

[Cydrome Leader]

the problem with rs-232 is isn't not even a real standard. If it works
with 6 foot cables, great, and most stuff does.

For long distances, using fiber to serial converters works great. It's not
cheap though. It does eliminate grouding problems and noise issues though.

[k...@attt.bizz]

It certainly is a standard, or if you're a pedant, a series of
standards. It's defined by the EIA (and probably a dozen others). It
works with cables a *lot* longer than 6', but there is a length-data
rate tradeoff.

>For long distances, using fiber to serial converters works great. It's not
>cheap though. It does eliminate grouding problems and noise issues though.

Where is fiber in the RS-232 standards? RS422 or RS485 makes a lot
more sense than fiber, here, but RS-232 should work fine.

[Jim Thompson]

On Mon, 14 Jan 2013 05:52:32 +0000 (UTC), Cydrome Leader
<pres...@MUNGEpanix.com> wrote:

When I designed the very first 1488/1489 chips in the mid-60's there
WAS a standard. Now almost anything that switches between two levels
gets called RS-232.

[John Fields]

---
There still is; it's now called: "TIA-232-F", but back then (1969) it
was called: EIA RS-232-C.
---

>Now almost anything that switches between two levels
>gets called RS-232.

---
Well, other than that it _does_ have to follow the signaling protocol.

--
JF

[Cydrome Leader]

a series of old standards nobody really follows. Some of the worst
offenders are those usb to serial dongles which struggle to work with
anything.

>>For long distances, using fiber to serial converters works great. It's not
>>cheap though. It does eliminate grouding problems and noise issues though.
>
> Where is fiber in the RS-232 standards? RS422 or RS485 makes a lot
> more sense than fiber, here, but RS-232 should work fine.

notice they keyword "converter". Since you'd locate them just a few feet
away from your equipment you mostly eliminate problems with cables and
what kludged, random version of rs-232 somebody tried to implement in your
devices.

The original poster mentioned machine tools, not a dumb terminal attached
to a minicomputer in the other room. Complete electrical isolation may be
a good thing to have in this case.

It's also likely some ebay optical converers could be cheaper than blown
out serial ports in the long run too.

[Cydrome Leader]

bingo.

I was fiddling with a BASIC stamp recently and there's some hack mode to
fake serial output using just +5 and 0V. It will run a "rs-232" interface
vacuum flourescent display with no errors, so far, but it's such a sketchy
hack, like most serial stuff these days.

I'd like to see how the +5 to 0 serial plays with a classic terminal like
an ADM5a or ADDS Regent 100.