SCSI SE/DIFF - Dumb question
Steve Loughran
OK, Dumb question time: Where can find info about the differences between
single ended and differential SCSI? Can you mix the two types of devices on
the same bus? Is it the bus that is different or just the terminators?
I have an Origin 200 (2 x external Diff SCSI Cards), a Baydel unit (Diff)
and a DLT4000. I`m not sure which the DLT is (SE or Diff) but I'm assuming
its SE. I did plug it into the spare SCSI DIFF card but it didn't detect it
at power up so I removed it pretty damn quick just in case.
Any help with my lack of basic SCSI understanding would be greatly
appreciated.
--
Steve
-------------------------------------------------
Steve Loughran, SGI/PC IT Manager
Sony Computer Entertainment Europe (Cambridge)
http://camsg001.millennium.co.uk/index.htm
Yamaha YZF1000R Thunderace
Team Waste - Where do you want to go wrong today?
Richard Masoner
"Steve Loughran" <Steve_L...@scee.sony.co.uk> writes:
> OK, Dumb question time: Where can find info about the differences between
> single ended and differential SCSI? Can you mix the two types of devices on
> the same bus? Is it the bus that is different or just the terminators?
The bus is different. In single-ended SCSI all the data lines share a
common ground. With differential, there's a pair of wires for each
signal, which gives increased noise immunity and thus longer cables.
--
Richard Masoner <rich...@bif.dgii.com> Digi International
Tony Izzi
Single-ended, the signal ground is relative to the HBA (host-bus adapter).
Differential, the signal ground is shared between both HBA's, therefore the
signal voltages can be carried greather distances.
APCON makes a box that converts from single-ended to differential
check: http://www.apcon.com/ under converters. I've seen them work on SGI's
in the past, but make no guarantees. your best bet is to get a single ended
card for the O200 if you still have and available PCI slot.
Tony
Steve Loughran wrote:
> Hi all
>
> OK, Dumb question time: Where can find info about the differences between
> single ended and differential SCSI? Can you mix the two types of devices on
> the same bus? Is it the bus that is different or just the terminators?
>
> I have an Origin 200 (2 x external Diff SCSI Cards), a Baydel unit (Diff)
> and a DLT4000. I`m not sure which the DLT is (SE or Diff) but I'm assuming
> its SE. I did plug it into the spare SCSI DIFF card but it didn't detect it
> at power up so I removed it pretty damn quick just in case.
>
> Any help with my lack of basic SCSI understanding would be greatly
> appreciated.
>
> --
>
> Steve
>
> -------------------------------------------------
> Steve Loughran, SGI/PC IT Manager
> Sony Computer Entertainment Europe (Cambridge)
> http://camsg001.millennium.co.uk/index.htm
> Yamaha YZF1000R Thunderace
> Team Waste - Where do you want to go wrong today?
--
Tony Izzi
Southeast Region Field Technical Support
Chantilly, VA
-- Live the wild life - have kids! --
Alexis Cousein
>
> You're correct, the DLT4000 is single-ended,
>
> Single-ended, the signal ground is relative to the HBA (host-bus adapter).
>
> Differential, the signal ground is shared between both HBA's, therefore the
> signal voltages can be carried greather distances.
That is slightly confusingly worded. In the differential signaling used,
the polarity between the two carriers determines the logic level --
there is no such thing as a "signal ground", as both wires carry a
voltage level (and when one carrier goes high, the other goes low).
The excerpt from the SPI standard's wording is:
All differential signals consist of two lines denoted +SIGNAL and
-SIGNAL. A signal is true when +SIGNAL is more positive than -SIGNAL,
and a signal is false when -SIGNAL is more positive than +SIGNAL.
Contrast this with Single-ended operation wording:
[Single-ended signaling] SCSI devices with power-on shall meet the
following electrical characteristics on each signal (including both
receivers and disabled drivers):
a) VIL (Low-level input voltage) = 0,0 V d.c. to 0,8 V d.c. (signal
true);
b) VIH (High-level input voltage)= 2,0 V d.c. to 5,25 V d.c. (signal
false);
Single-ended SCSI *also* channels signal ground wires (always at the
reference "0V" voltage level) from SCSI adapters over wires (those of us
who've had the pleasure of finding grounding current loops between
devices know this ;) ). They aren't used to channel information, though,
just to make sure all devices share the same voltage reference.
Differential signaling is more robust because the voltage swing
(measured between signal wires) when changing the logical signal value
is doubled, because there is no "grey zone" for which the signal value
is undefined, and because noise tends to influence both +SIGNAL and
-SIGNAL voltages in the same direction.
--
<standard disclaimer: these are my personal views, not SGI's>
Alexis Cousein a...@brussels.sgi.com
Systems Engineer SGI Belgium