Partial conversion of RS-232 to RS-422 using a Comercial product

Discussion in 'General Electronics Chat' started by thomaskuhn, Oct 9, 2012.

  1. thomaskuhn

    Thread Starter Member

    Jan 21, 2009
    20
    0
    Hi Everyone,

    I have an interesting problem that I would like to share and see if anyone knows if this will work. I have two devices communicating with each other via serial interface. It is not really a true RS-232, becasue there is a limited set of signaling. Device 1 transmits unbalanced serial data, Receives balanced serial data, and has one signaling output (a Push-to-Talk to be specific).

    Device 2 Transmits Balanced serial data, receives balanced serial data, and uses RTS to handle the PPT input.

    Due to the long distance between device 1 and device 2 (>110ft) I would prefer balanced signals throughout. The only signal that would have to have be converted to balanced would be device 1's Tx data. I would like to use an off the shelf 8 channel 232 to 422 converter. and my goal would be to use only one of these with no external bypassing of the device. When I say bypassing, I mean have a signal wire (PTT for instance) go from device 1 to device 2 outside of the inline cable converter.

    So my question is: is there a simple way to wire to the converter in which I can pass the three signals (PTT, TX (Bal) A, and TX (bal) B) through this device without any modifications? perhaps put the signal on one balanced pin and ground the other? Something like that?
     
  2. panic mode

    Senior Member

    Oct 10, 2011
    1,320
    304
    i am unsure what you really have there. first of all i am not even interested to look at some circuit without values and part numbers. it looks like Device1 has true RS232 port, then you are adding MAX232 kind of device to convert that to TTL then use SNLS373A like device to create 485/422 signals which are then carried over long wires to Device2.

    now about confusing part:
    1. power seem to be taken from Device2 side (long distance to electronics located neart Device1)
    2. there are no terminating resistors on 422 line, you need them on both ends of each balanced line
    3. what kind of things you really have there, how come your have mix of unbalanced and balanced Tx/Rx channels? if you want to connect Rx of Device 1 to Tx of Device2, just use pair of wires and terminating resistors.
     
  3. thomaskuhn

    Thread Starter Member

    Jan 21, 2009
    20
    0
    Hi,

    Thanks for the reply. Things were changed up a bit. A different COTS part was speced out. IT is a B&B electronics 4 channel 232 to 422 converter. I am attaching the schematic. It is currently working, but I currently do not have the terminating resistors in place.

    The equipment in question is very old 1960's and 1970's military electronics, so there are some strange communications protocols going on.
     
  4. thomaskuhn

    Thread Starter Member

    Jan 21, 2009
    20
    0
    Could someone tell my why, when I have a +12V DC (Goes to Ground on Push-To-Talk) coming into the RTS on the RS-232 side, I find that it gets shorted somehow. It I measure the voltage once I hook up the adapter, the 12V goes to 0V. Why is this?
     
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