SCSI cable for digital and analog signals?
Christopher R. Carlen
We have obtained a DT3010/268 DAQ card from DataTranslations. I wasn't
consulted on its purchase. This model has both digital and analog
signals carried on a single 68-pin SCSI cable, with some more digital
signals on another 68-pin SCSI cable.
They have a different model, the DT3010/32 that provides all the
functinoality we want, but separates the analog and digital signals on
two different cables, a 68-pin SCSI for the digital, and another 50-pin
something for the analog.
I prefer the second case, since I am concerned that crosstalk will occur
with the /268 model, when outputing digital signals simultaneously with
inputing analog signals. I am trying to decide whether to swap for the
/32 model, before I commit to laying out a PCB that will break out the
signals to panel BNCs for our lab researchers to get their paws on.
So I have two questions:
1. What is your experience with multifunction DAQ hardware that has
digital and analog signals on the same cable? Are there crosstalk
problems that cannot be mitigated in these systems?
2. What exactly is inside a SCSI cable? I know it is designed for
digital transmission, with matched terminating impedances at the end of
the transmission lines, so there must be something about its
construction to create TEM lines, and certainly it wouldn't be desired
to have alternate modes of transmission available against the other
lines, so is it constructed of twisted pairs, shielded pairs, a bunch of
little coaxes, or what? Ie., what is inside a SCSI cable, or how does
it work?
Thanks for input.
Good day!
--
____________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crc...@sandia.gov
Mario Trams
> We have obtained a DT3010/268 DAQ card from DataTranslations. I wasn't
> consulted on its purchase. This model has both digital and analog
> signals carried on a single 68-pin SCSI cable, with some more digital
> signals on another 68-pin SCSI cable.
>
> They have a different model, the DT3010/32 that provides all the
> functinoality we want, but separates the analog and digital signals on
> two different cables, a 68-pin SCSI for the digital, and another 50-pin
> something for the analog.
>
> I prefer the second case, since I am concerned that crosstalk will occur
> with the /268 model, when outputing digital signals simultaneously with
> inputing analog signals. I am trying to decide whether to swap for the
> /32 model, before I commit to laying out a PCB that will break out the
> signals to panel BNCs for our lab researchers to get their paws on.
>
> So I have two questions:
>
> 1. What is your experience with multifunction DAQ hardware that has
> digital and analog signals on the same cable? Are there crosstalk
> problems that cannot be mitigated in these systems?
I'm not really an expert here and can't give you a quantitative answer
on that. Finally, this depends one some parameters that you haven't
mentioned so far.
What I can tell you is that it depends on the rise and fall times of
your digital lines and the balance of data lines vs. ground lines.
The lower the slew rates the better and the more ground lines the
better. In best case one would use differential signaling and twisted
pair for the digital lines. Using reduced slew rate differential
drivers is a plus.
Perhaps you should ask the manufacturer of this equipment what he
is telling you about this issue. If this is a good manufacturer,
he made this design with care and testing and everything should
be alright.
> 2. What exactly is inside a SCSI cable? I know it is designed for
> digital transmission, with matched terminating impedances at the end of
> the transmission lines, so there must be something about its
> construction to create TEM lines, and certainly it wouldn't be desired
> to have alternate modes of transmission available against the other
> lines, so is it constructed of twisted pairs, shielded pairs, a bunch of
> little coaxes, or what? Ie., what is inside a SCSI cable, or how does
> it work?
There is nothing special with SCSI cables. Inside boxes standard flat
ribbon cables are being used - AWG28 for the older 8 bit bus and some
cable with a shorter grid (I don't have the designation in mind) for
the newer systems.
With differential SCSI it is the same only that each SCSI signal pair
is twisted.
Regards,
Mario
Bill Sloman
I've got a nasty feeling that the answer to your question depends on
the flavour of SCSI involved. IIRR the earlier versions had
single-ended unbalanced signals in a ribbon cable with every second
wire a ground, while the later versions used balanced low-voltage
digital signalling with rather fewer ground connections - I haven't
had to worry about SCSI for at least five years now, so I've forgotten
most of the details, and SCSI may have evolved even further.
------
Bill Sloman, Nijmegen
Mario Trams
> I've got a nasty feeling that the answer to your question depends on
> the flavour of SCSI involved. IIRR the earlier versions had
> single-ended unbalanced signals in a ribbon cable with every second
> wire a ground, while the later versions used balanced low-voltage
> digital signalling with rather fewer ground connections - I haven't
> had to worry about SCSI for at least five years now, so I've forgotten
> most of the details, and SCSI may have evolved even further.
There has nothing changed here. When you go in a hardware store
and buy a SCSI device you will receive a single-ended one - very
likely with such a 68pin fine-pitch connector (that's 16bit SCSI).
Differential SCSI is primarily used for a ultra-expensive and reliable
server hard disks.
Regards,
Mario
Tim Shoppa
> consulted on its purchase. This model has both digital and analog
> signals carried on a single 68-pin SCSI cable
I will remind you that "digital" signals are still analog signals, just
interpreted (and maybe driven) differently.
> 1. What is your experience with multifunction DAQ hardware that has
> digital and analog signals on the same cable? Are there crosstalk
> problems that cannot be mitigated in these systems?
If you have crosstalk between digital and analog signals in the same cable,
you'll have the same crosstalk between analog and analog signals too.
(Assuming the analog signals have similar risetime, etc.)
A lot depends on impedance levels, single-ended vs differential, etc. These
are all issues for both analog and digital.
> 2. What exactly is inside a SCSI cable? I know it is designed for
> digital transmission, with matched terminating impedances at the end of
> the transmission lines, so there must be something about its
> construction to create TEM lines, and certainly it wouldn't be desired
> to have alternate modes of transmission available against the other
> lines, so is it constructed of twisted pairs, shielded pairs, a bunch of
> little coaxes, or what? Ie., what is inside a SCSI cable, or how does
> it work?
A good SCSI cable is a bunch of twisted pairs and an overall shield.
Tim.
Ian Buckner
"Tim Shoppa" <sho...@trailing-edge.com> wrote in message
news:bec993c8.03042...@posting.google.com...
a pair of pairs per screen, I forget) as well as overall screening. We
are
not using it for SCSI, this was for another aplication. You can also
get a ribbon
cable made up of miniature coax, but that is fairly pricey.
Regards
Ian
Winfield Hill
>
> We have obtained a DT3010/268 DAQ card from DataTranslations. I wasn't
> consulted on its purchase. This model has both digital and analog
> signals carried on a single 68-pin SCSI cable, with some more digital
> signals on another 68-pin SCSI cable.
Why do you say it's a "SCSI cable?" It's likely that DT's 68-pin
connector and cable was modeled after National Instrument's version,
which is a custom designed cable that has fully-separate shielded
analog and digital portions. E.g., see this document's first page,
http://www.ni.com/pdf/products/us/3daqsc221-228_370-373.pdf
AFAIK it's a rather special cable that cannot be usefully compared
to an ordinary SCSI cable. BTW, we have been using these for several
years with good success.
Thanks,
- Win