Old Lab Equipment rs423

Discussion in 'General Electronics Chat' started by jamus, Aug 18, 2015.

  1. jamus

    Thread Starter Member

    Feb 11, 2013
    I found an old piece of lab equipment in the dumpster.
    Found along with it was a Compaq 386s that has a version of MSdos installed as well as the software for the instrument.
    The lab equipment is very obscure, chemistry related, and from 1987. There are no manuals available.

    The interface for the instrument is an unlabeled db25 female connector. I originally thought it was a parallel port connector, however when I took it apart and looked at the connected pins and their drivers, things did not match up.

    The datasheets for the line drivers and receivers told me that they were specifically designed for rs423. This is a weird and kind of obscure standard.
    For starters, wikipedia states that the standard has no actual standard pinout! Weird eh? I did find a pinout on a website, but it did not match mine.

    From looking at various webpages, I determined that rs423 is single ended and considered backwards compatible with rs232. Basically it is like a single ended version of rs422, allowing 10 devices to be accessed on a bus. I also found a reference for a Sun Microsystems Server that listed the DB25 rs232 port as being RS232/RS423 compatible.

    My decided course of action was to make an rs232 9 pin to rs232 25 pin adapter and pray for success. Tracing the various pins of the electronics I saw that the drivers and receivers seemed to agree at least somewhat with the pinout of an rs232 25 pin. I was using all my drivers and receivers and I wasn't connecting anything wrong.

    Anyway, I wasn't in luck. Any suggestions on how to proceed?
  2. RichardO

    Well-Known Member

    May 4, 2013
    What are the part numbers of the drivers and receivers? If the are for RS-232 then the jumpering of RTS, CTS and the like is probably wrong in your adapter. Give us a drawing or table showing how you wired your adapter...
  3. jamus

    Thread Starter Member

    Feb 11, 2013
    The drivers and receivers are for RS423. I have read that the standards are backwards compatible. Here are the part numbers and datasheets.

    9636A - I have two of these

    Am26LS32B - four channel receiver, positive inputs are grounded on the board

    I wired my adapter using the attached image as a reference.

    Here are my observations from testing the board:
    1 - NC
    2 - Pin 7 driver2 (output A)
    3 - Pin 15 reciever (input B-)
    4 - Pin 6 driver2 (output B)
    5 - Pin 9 receiver (input D-)
    6 - Pin 1 receiver (input A-)
    7 - Pin 4 driver2 (Ground)
    8 - NC
    9 - NC
    10 - NC
    11 - NC
    12 - NC
    13 - NC
    14 - Pin 6 driver1 (output B)
    15 - NC
    16 - NC
    17 - NC
    18 - Somewhere way off in space that I can't find
    19 - NC
    20 - Pin 7 driver1 (output A)
    21 - NC
    22 - NC
    23 - NC
    24 - NC
    25 - NC
    Last edited: Aug 18, 2015
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008

    It's chemistry equipment. Do you even know what it did in those days? Maybe in that environment there may be some use, but from an electronics standpoint, it is worthless.

  5. jamus

    Thread Starter Member

    Feb 11, 2013
    I am working as an electronics technician in a chemistry lab this summer. This is a side project that I am doing for fun. I find it fun to get old equipment working again. The equipment is obscure in its purpose, but similar machines are still being made today. Once it is working it will be put to use.

    Perhaps what you wanted to know what it is I am actually working on. The equipment is a spectrophotometer. It is used to measure different qualities of how a substance interacts with light. Here is a similar one to mine. [​IMG]
    Last edited: Aug 18, 2015
  6. MaxHeadRoom


    Jul 18, 2013
    The original RS232 COM port was 25pin, RS423 I think was a forerunner of RS485, multi drop?
    In fact the original 25p RS232 used the same pins as you show!
  7. jamus

    Thread Starter Member

    Feb 11, 2013
    Maybe I just have to tweak my cable... Is there any reason why the cable diagram that I posted wouldn't work if the port was an RS232?
  8. jamus

    Thread Starter Member

    Feb 11, 2013
    I think I have an idea... The device probably utilizes a discrete serial driver chip such as the Intel 8251A. I remember it having a lot of Intel chips. It just didn't occur to me because I am used to modern digital stuff where everything would be taken care of in software. I will look into this tomorrow.