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RS422 flipping bits

Discussion in 'Electronic Basics' started by [email protected], Oct 26, 2006.

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  1. Guest

    I suspect a device I have is flipping bits before it sends out RS422
    signals. I sent 3 capital A's out of it, and the oscilloscope trace is

    I am using 1 start bit, 8 data bits, even parity, and 2 stop bits. So,
    does anyone know if the bits are flipped?
  2. Lord Garth

    Lord Garth Guest

    A mark is supposed to be the lowest level while a space is the highest...
    the trace looks correct to me.

    If you need to invert the data, a single transistor and a couple of
    can do the job.
  3. Guest

    The letter A is 1000 0010 (least significant bit first).

    So, are you saying it's inverted?
  4. Chris

    Chris Guest

    RS-422 and RS-485 are supposed to have two outputs, A and B. A is
    supposed to be non-inverting, and B is supposed to be inverting. You
    didn't label voltages, so I'd assume you're looking at A.

    If you have a straight RS422-to-RS422 connect, If you put a logic 1
    (5V) into the driver, and then you connect A of the driver to A of the
    receiver, and B of the driver to B of the receiver, you will see a
    logic 1 (5V0 at the output of the receiver.

    If your situation is more complex, possibly you could grant us a few
    more details. Still waiting on a description of whether this is an
    existing setup or you're cobbling something up.

    Since you seems to be new to this, and you're also a Google Grouper (so
    am I), you should know that it's considered good form to bottom post
    (put your answer *under* the copy of the prior post). You should please
    check out Google Groups Help Topic "What's good 'netiquette' when
    posting to Usenet?"

  5. john jardine

    john jardine Guest

    Yes. Those "A" bits have been flipped.

    Looks like you are using the UART on a micro to format the send characters
    and from the way the LSB's are being sent out first, you're also using a
    UART to receive them.

    If so, the sending UART will be -absolutely- flipping those "A" bits. It
    knows only of RS232 and nothing of RS422, hence assumes it is sending data
    to a Max232 type inversion chip. You can see this from the UART pin being at
    +5V in a 'no data' condition.
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