Re: RS-232 DB25 MUX connection question

[Lewis Gardner <lgardner@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

Robert Green wrote:
> I've got a MUX with a DB-25 connector on the back.  The manual tells me that
> the pins are arranged as follows:
>
> 01  -  Alarm Input 1
> 02  -  Alarm Input 2
> 03  -  Alarm Input 3
> 04  -  Alarm Input 4
> 05  -  Alarm Input 5
> 06  -  Alarm Input 6
> 07  -  Alarm Input 7
> 08  -  Alarm Input 8
> 09  -  RS-232 RX Input
> 10  -  RS-232 TX Input
> 11  -  RS-232 RX Output
> 12  -  RS-232 TX Output
> 13  - VCR Trigger Input
> 14  -  Alarm Input 9
> 15  -  Alarm Input 10
> 16  -  Alarm Input 11
> 17  -  Alarm Input 12
> 18  -  Alarm Input 13
> 19  -  Alarm Input 14
> 20  -  Alarm Input 15
> 21  -  Alarm Input 16
> 22  -  Alarm Hold Input
> 23  -  Alarm Output Common
> 24  -  Alarm Output NC
> 25  -  Alarm Output NO

> Does anyone have any suggestions how to wire the MUX pinout above so that I
> can communicate with it via a 9 or 25 pin PC serial port?

RS-232 only requires 3 wires TX RX and ground for non-hardware
handshaking communication. On a 9 pin connector those are on pins 3, 2
and 5 respectively. No standard serial cable is going to work with the
pinout you have listed above.

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.

In an earlier post you mentioned "these MUX boxes have a RS-485 port to
connect to each other". Is it possible that pins 9 thru 12 are actually
RS-485 which uses 4 wires?

Do you have the serial protocol (baud, stop bit, parity and command
structure) for this box?