Custom 3pin to 9pin D-Sub Pin configuration

Discussion in 'General Electronics Chat' started by dmarciano, Apr 19, 2012.

  1. dmarciano

    Thread Starter Member

    Oct 10, 2007
    21
    0
    I'm building a special downloader for an instrument we use at our company. The instrument came with a cable that has three pins in a triangle at one end (which plugs into the instrument) and a D-SUB female connector at the other end (which you can use a RS232-USB adapter to connect it to a computer). I used a multimeter to check which pins are connected at each end.

    The lower left pin connects to the D-SUB Ground pin, the top pin connects to Pin 3 (TXD), and the lower right connects to Pin 2 (RXD). What I can't figure is if the data on the RXD pin is going from the instrument to the computer, or from the computer to the instrument; same goes for the TXD pin. I'm thinking that the RXD is into the instrument and the TXD is out from the instrument. Would this be the correct assumption?

    Switching the cables after I build the circuit wouldn't be very difficult at all, but I would prefer to know beforehand. Thanks in advance for any help or info.
     
  2. hexreader

    Active Member

    Apr 16, 2011
    250
    82
    Use a DMM (voltmeter) to look for approximately -9V on the TXD and RXD pins (with respect to ground)

    Connect the -9v (approx) on one side to floating (maybe 0v approx) on the other side.

    Usually the -9v indicates output (driven) and around 0V suggests input.

    Has always worked for me. Hope it works for you.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    The convention is the PC is designated to be DTE - Data Terminal Equipment.
    The DE-9P connector on a PC COM port is wired as follows:
    Pin 2 = RXD, input into the computer
    Pin 3 = TXD, output from the computer

    Sometimes the handshake pins need to be wired (jumpered) in the plug to satisfy the handshake requirements.
     
  4. dmarciano

    Thread Starter Member

    Oct 10, 2007
    21
    0
    Now that I think about it, that makes sense since the equipment can't do anything without commands from the computer (computer initiates downloads, resets, etc.) I think that is what I was missing. Thanks
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    If you need to know which handshake lines have to be jumpered I can provide that.
     
  6. dmarciano

    Thread Starter Member

    Oct 10, 2007
    21
    0
    Nothing will have to be jumped. I'm not going to connect through the RS232 but directly to the 3-wires. I only did the test on the RS232 connector to try and determine which lines were each of the 3pins. Appreciate it though.
     
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