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RS485 -> RS232 -> V34 -> RS232 -> RS485

Discussion in 'Electronic Design' started by amnw14545, Nov 11, 2005.

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  1. amnw14545

    amnw14545 Guest


    Apologies if this isn't the correct place to post this. It's not
    strictly an electronics design issue. In addition, it may be a stupid


    I have an RS485 transmitting device at L1.
    I have an RS485 receiving device at L2.

    I get from L1 to L2 by the following steps:

    At L1 I convert the RS485 data to RS232 data and feed it into a modem.
    At L2 I read the data via the modem and convert it from RS232 to RS485

    At L1 I mismatch the TX+/- lines (ie. L1 device TX- connects to RS485
    converter TX+, and L1 TX+ to RS485 converter TX-).

    At L2 I mismatch the TX+/- lines (ie. L2 device TX- connects to RS485
    converter TX+, and L2 TX+ to RS485 converter TX-).

    Will L2 receive the data transmitted by L1 correctly?

    That is, will both mismatches cancel each other out?

  2. Guest

    Hmm.. interesting, my hunch (from field experience) is it would work.
    But I'm not sure. Maybe the first and last bytes gets corrupted. Anyone
    knows for sure?
  3. Hi,
    I would expect, that it won't work, because the Modem needs to be configured
    to dial... and with the wrong polarity of signals most modems wouldn't
    understand, what you want.

  4. My arse is itchy, if I scratch it will it stop itching?
  5. amnw14545

    amnw14545 Guest

    Ah, but the modems are on a private line and dial/answer regardless of
    data being present...
  6. amnw14545

    amnw14545 Guest

    Interestingly, if I interface to the RS232 at L2, I have to right bit
    shift and then negate the data for it to look similar to what
    originates at L1...

    So yes, it looks like the mismatches may be cancelling each other out.

    I'd best fix that then ... since I need to tap off the RS232 data at

    Locations are (very) remote and outside so thought I'd better check
    before sending someone into the wind + rain to open the cabinets and
    swap both sets of wires.
  7. amnw14545

    amnw14545 Guest

    I guess this depends both upon the size of your arse and the vigour
    with which you scratch it...
  8. Hello,
    This depends on the modem and its settings.
    If the modem is set up for example with error correction, it will look at
    the asynchronous RS232 charcater framing (start bit, stop bit) which is not
    valid any more if you change polarity. In this case you won´t get the
    correct data on the other end.
    If the modem uses a direct mode without any data framing (=it directly
    modulates the data bit by bit) it will work. Unfortulately the other case
    is the most usual.

  9. Jasen Betts

    Jasen Betts Guest

    only if you are using synchronous modems.

    if using asynchronous modems (eg, common consumer modems) the mis-match
    will garble the data. because the start bit won't look like a start bit.
    and an idle period will look like a break.

    you may get good data some of the time (data dependant) if using async
    modems that run the same DCE and line bitrates but modems with any sort
    of compression, bit-rate conversion, or error correction will probably
    garble your bitstream beyond recognition. (in consumer async modems
    this essentially rules out anything on the consumer market capable of
    faster than 9600bits per second, and a good number of 1200 and 2400
    capable modems,

    If you only send odd (or is it even?) bytes it could also work - atleast
    in the middle of the packets. ends could still be garbled.

  10. amnw14545

    amnw14545 Guest

    Thanks, the modems are 33.6K Multitech 2834BLK models that have been
    set to synchronous mode via the switches on the side and run at

    I'll get the jumper settings on each modem sometime Monday, and we'll
    see what happens when source and dest wires are reversed.

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