Re: RS-232 DB25 MUX connection question

[Lewis Gardner <lgardner@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

>>>09  -  RS-232 RX Input
>>>10  -  RS-232 TX Input
>>>11  -  RS-232 RX Output
>>>12  -  RS-232 TX Output

>>My guess is that on pins 9 thru 12 two of those pins are grounds and the
>>other two are TX and RX. The first check would be to probe those pins
>>with a multimeter and see if any of 9 thru 12 were connected together.
>>If they are those are likely the ground pin.
>
> Would you test for continuity with the unit off?  I'm only marginally
> competent with electronics, I'm afraid.  I considered looking for voltage on
> the pins with the unit running, but wasn't sure which pin to use as ground.
> I was leaning towards pin 23  -  Alarm Output Common but it's really just a
> guess.  I suppose I could pop the case and try tracing the pins in question
> by eye.  <sigh>

For the first bit of testing I would have the unit off and check for
continuity between any two pins in the group 9 thru 12. If you find
continuity between any of those two pins they are most likely ground.

If there is no continuity between any of the pins you will need to open
the thing up and start tracing. The ground pin(s) will likely go back to
the biggest trace on the circuit board. I would stick to pins 9 thru 12
for serial communications until I was sure something else was required.

Once you have found the ground then sorting out the RX and TX is trial
and error since "TX Input" and "RX Output" are fairly meaningless. Many
serial devices will transmit a string when they power up. That is how I
have found many a TD pin and the baud, data bit, parity and stop bit
settings.

Good luck!