Serial transceiver question

Discussion in 'General Electronics Chat' started by vortmax, May 1, 2013.

  1. vortmax

    Thread Starter Member

    Oct 10, 2012
    I'm working with some RS232,485 and 422 transceiver chips (MAX232e, MAX13448e and MAX13487e) and am experiencing unexpected behavior.

    Based on prior experience with using MAX232 chips with a micro, I was expecting the RX line (receiver output) on the chips to be active high (idle low)....and since they are fail-safed I was expecting that if I powered them up with no connection, they would idle low, but instead they are all holding the receiver output pin high.

    I went back and studied the data sheets and this is apparently the proper behavior.

    So now I'm confused. Are these inverting drivers (active low)? If that's the case, why have I never had to invert the output to communicate with a micro?
  2. MrChips


    Oct 2, 2009
    I am not sure what you mean by active low with respect to an RS232 transceiver.

    The RS232 protocol defines a MARK as low voltage and SPACE as high voltage. The idle condition is MARK, i.e. low voltage.

    RS232 transceivers are inverting.

    UART, USART, SCI, ACIA etc output TTL levels that are high on MARK and low on SPACE. The idle condition is MARK (high voltage).

    Hence you connect the TTL signals of the UART etc. directly to the RS232 transceiver without any need to invert.
  3. vortmax

    Thread Starter Member

    Oct 10, 2012
    Okay....I guess I was just thinking backwards. I was thinking that the UART idle state was low. thanks