Connect with us

RS422 flipping bits

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

Scroll to continue with content
  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
    at:

    http://www.coolgroups.com/AAA.JPG

    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
    resistors
    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?"

    http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

    Cheers
    Chris
     
  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.
    john
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-