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Discussion in 'Electronic Design' started by [email protected], Oct 8, 2006.

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  1. Guest

    In RS-232, are the start and stop bits always logical 1's?

  2. feebo

    feebo Guest

    generally the start bit is a high and the stop bit(s) is low so you
    can definately see the start bit.

    in idle, the output will be low (usually left there after the last
    character transmitted, you see the line go high and wait for 150% of
    the bit width - this waits for the start bit to end and you should be
    half way into the first bit of "real" data. you then sample and wait
    100% bit width (so now you should be bang smack in the middle of each
    bit as it's transmitted) for 8 cycles (if u r doing 8 bit data) you
    then check to see the line is low - if not, you have a transmit error.

    each time you sample a bit, you right shift (RS232 is LSB first) it
    into a register so that after 8 cycles, you have one byte of info.
    Stash that somewhere and repeat the above for each character.

    If the stop bit(s) were not low, you wouldn't be able to see the start
    bit go high. If you are transmitting 0xFF, the only low bit in the
    whole 10 bits (if using 8N1 - 8 data bits, no parity, 1 stop bit -
    very common format) is the stop bit so you need to have a low in order
    to see the next high-going start bit.
  3. John B

    John B Guest

    The OP asked about logic levels and not voltages. In RS232 anything in
    the range +3V to +24V represents logic 0 and -3V to -24V represents
    logic 1. The area between +3V and -3V is undefined. So the start bit is
    logic 0 and the stop bits/line idle are logic 1.
  4. PeteS

    PeteS Guest

    Well, to be somewhat pedantic, RS232 doesn't really specify the data
    format; it specifies the signalling levels :)

    That said, JohnB is completely correct.

    For the OP - easy enough to look up. All control signals are active
    when logically '0' on RS232. Start -> logical 0, Stop -> logical 1.


  5. jasen

    jasen Guest

    assuming you mean asynchronous RS232: (like PCs do)

    start is a space (logical 0, +3 to +15V)
    stop is a mark (logical 1 -3 to -15V)

    idle is a mark.

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