How to sense a disconnected cable on uart

Discussion in 'Homework Help' started by mgx2000, Jun 8, 2004.

  1. mgx2000

    Thread Starter New Member

    Jun 7, 2004
    I am not a student, and I am new at messing around with circuits and have a newbie question for a hobby project.
    I programmed a microcontroller to talk to an external device using an ansync serial interface, working on CMOS levels. My program needs to be able to detect when the cable of the external device is disconnected.
    My first idea was to have a resistor from the RX going to ground, so that in case the cable is disconnected, I would get all logical spaces and therefore a "modem break" on my serial port. The problem with that approach is, if the resistor value is too high (>500 Ohm), the microcontroller will not see the spaces (I guess the input is still considered high?). On the other hand, some external devices don't like it if the resistance is too low (<1000Ohm) and refuse to operate.
    Am I missing something obvious, or is there a better way to detect a disconnected serial connection?

  2. beenthere

    Retired Moderator

    Apr 20, 2004

    Try adding another line to the cable out to your peripheral. It does'nt need to do anything but pull down to ground if it's plugged into the device. Your microcontroller can sense this line with an otherwise inused input pin.

    About the only other methodology is to make the peripheral capable of sending back a response to a uniqu query from the microcontroller. This is probably more trouble than it is worth.

    There's another reason to miss the good old days. A full serial interface used all 25 pins on the connector (or at least assigned functions to them). That would make it easy to produce a condition like a mark on secondary clear to send.