SLC RS232 led

What bit lights the RS232 led on SLC5/03, 05? I'm running a VB app using Channel 0 and want to generate an error message if communication stops. I thought I could use that bit, but I'm unable to find it.

Interesting question.

There are status bits for 'Channel Communications Active'... S:33/4 for channel 0, S:1/7 for channel 1 on an SLC5/05. I don't know how they respond though. (Look in "Processor Status" for additional comms status bits).

My normal approach is just to start a timer, and reset it every valid message transaction. As long as comms are running, the timer stays reset. If comms fail though, the timer times out, latches a fault bit, and I use that for the failure detect.

Heartbeat

If you are using OPC you could use the "health of communication" bits.
Normally though you would use a heartbeat bit exchange. This is explained in some AB manuals for ENI (Ethernet) or DNI (devicenet)modules. Basically you provide a signal to set a bit in the PLC.
Within the ladder code this bit will set another bit (coil), which you will then monitor. If set you know this beat was OK. Subsequently
you issue a command to reset the same bit and monitor the output bit for being reset. If reset all is OK. Then you continue again setting the bit and so on and so on. Ladder logic is processed when CPU is in the RUN mode and that's why you need to have that one line of ladder code otherwise you could just use the same bit.