I am looking for a command to use in my ML1100 and 1500 that more or less says "I'm OK" and "I'm powered up".

I want a bit to come on when those conditions are true. Is there an instruction that will accomplish that?

Thanks.

Bit S:1/15 is the First Pass Bit it comes on when the first scan is started. It is only on for the very first scan after power up or transistion from Program to run Mode.


I use this bit to reset my machines alot so everytime I start I start the same way.

I have used the Processor Faulted bit when interfacing with HIM's.

That bit is set and can be read from outside the PLC when a fault occurs.

S:1/13

How about an always on bit? --------------------( ) ????

I've also seen a watchdog toggle bit used.

PLC A sets output which is wired to an input on PLC B.
PLC B sees input and sets its output to acknowledge, B's output is wired back to an input on A.
A sees the acknowledge input and turns off output.
B sees the input toggle and clears its output to acknowledge.
A sees the acknowlege input go false and set its output on to acknowledge.
.
.
.
repeat forever.

Both PLCs run a timer which is reset each time it receives a change of state on the acknowledge input.
If the timer times out then the PLC takes appropriate action.

Yes we use a similar arrangement as a communications "heartbeat" to verify that communications have not been lost and everyone is "healthy".