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50ft SCSI cable?

By Eddie_42
Sep 14, 2009
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  1. Hello,

    I work in an R&D lab and 'the man' has decided we are building a new lab between my control station and the current lab.

    Long story short, I need a 50ft SCSI cable, 50 pins. else i cant separate the control station. There is nothing major being sent through the cable, just a signal to close a rely switch. I have a NI DAq and it outputs to this scsi cable. The cable is then plugged into a circuit board with 24 relays that operate valves.

    I am having an difficult time finding such a device since this cable is mostly for in-tower computer use.

    Would anyone have a suggestion of where i could get this cable, or at least the ends that i can splice on my own wires?

    Again, this is not a data transfer rate issue, just a valve open/close signal. I dont think length is a factor.

    thanks
  2. strategic

    strategic TechSpot Paladin Posts: 1,274

    You'll have to visit your local computer repair shop. They should be able to make any cable for you.
  3. jobeard

    jobeard TS Ambassador Posts: 13,406   +314

    see this faq and search for length

    Max length for SCSI-I is 6 meters

    Code:
    QUESTION: How long can my SCSI bus be?
    ANSWER From: Gary Field (gfield@grcelect.ultranet.com)
    
    The SCSI length limits are based on the speed of the fastest device
    attached to the bus.
    
    Here's a table which shows the limits:
    
    Speed of FASTEST device    Max. single-ended bus length  Max. HV Diff. bus len.
       5 MHz (SCSI1 synch.)         6 meters                    25 meters
      10 MHz (SCSI2 FAST)           3 meters(not rec.)          25 meters       
      20 MHz (Ultra or Fast20)      1.5 meters(not rec.)        ?
    
  4. Eddie_42

    Eddie_42 TS Rookie Topic Starter Posts: 213

    those max lengths are to ensure the transfer speeds. My speed isnt hyper critical, so i can suffer a little more lag.

    i just keep perusing the web, thanks
  5. jobeard

    jobeard TS Ambassador Posts: 13,406   +314

    One reason that a SCSI channel is terminated at both ends
    (one at the controller and the other at the physical end) is to reduce 'reflection'.
    The cable length acts very much like the string on a violin, the shorter the string, the higher the pitch.

    RF data (and our channels are operating just like RF) is very sensitive to
    • even harmonics, where the reflection is subtractive and reduces signal strength and introduces errors on the media
    • odd harmonics, where the reflection is additive and swamps the receiver, again intruducing errors.
    there's more to the length than just 'speed'
  6. raybay

    raybay TS Evangelist Posts: 10,716   +6

    In my experience, you will "suffer" as I don't think the quality of the cables and connectors has become any better in the past 12 years... I don't need to use them any more, but back then test after test, setting after setting showed us that 50 feet was way too long...
    Optimum, as I recall stopped at 16 feet, with rapid fade after that, so that anything over 30 feet was useless unless you were there to babysit it.
  7. raybay

    raybay TS Evangelist Posts: 10,716   +6

    You may not need it, but since they don't work for anybody at that length, why make one, or try to sell it, or try to support it after you sell it.

    You can always to to L-Com or other wiring vendor. I am sure they will put together the best possible one for you.
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