RS232 to 2 TTL devices

Discussion in 'The Projects Forum' started by alyeomans, Oct 6, 2015.

  1. alyeomans

    Thread Starter Member

    Sep 13, 2010
    Dear All

    I am going a bit cross eyed with determining if my circuit will work as planned. You input is needed to set me right.

    The circuit purpose is to take an RS232 input and output and level convert/invert to 3.3V TTL serial. On the TTL side there are two devices: one is a bluetooth device for transmitting the data. And the the second a Microchip MCU to listen and sometime interrupt communication to sent status data - as such is priority.

    The uartRX constantly listens to the bluetooth received data. When the MCU wants to transmit it actuates CommsEN to close off the transmitted data to H1 and then outputs TTL data to the bluetooth TX.

    rs232 to ttl.png

    I am unsure on the impedances on the TTL BT and uart pins or the rules used to govern these kind of connections - i.e. TX drives the line idle high.

  2. grahamed


    Jul 23, 2012

    Multidrop Rx is OK but not Tx.
    Usual method is to use a diode-OR circuit. Place a diode in series with each output so it is not conducting when the output is quiet. Tie the (non-stripey) ends of the diodes together, connect to R2, and pull the junction to the rail through 1k or so.

    If memory serves the TTL will sit high and the RS232 sits negative so your invertors are correct
    alyeomans likes this.
  3. alyeomans

    Thread Starter Member

    Sep 13, 2010
    Thanks Grahamed. That helped. I finished up with the following circuit some resistor values need to be tweaked but it works.

    rs232 to ttl v2.png