multiple PLC hearbeat

[brucek]

 Is there a standard for doing a heartbeat between 2 or more PLC's? I'm thinking of just doing a ST4 (1 sec) in the Primary PLC and put it in peerlink. Then have a Timer in the Secondary PLC of say 5 seconds and have that bit in the peerlink enable the timer. If the Timer Done bit is ever on then it shuts down the motion outputs in the secondary PLC.

[franji1]

No standard, but you have the right idea.

You could put DST22 (current time as 1970 Epoch value, the number of seconds since Jan 1, 1970) into first 2 WORDS of a PLC's PEERLINK block (e.g. PL0/1 as PL0:SD, PL16/17 as PL16:SD, PL 32/33 as PL32:SD, etc.).

What's good about DST22 as the "heart beat", is that it is a numeric value, not a bit, so there's no chance of hitting some "resonating" flaky comm sequence where the single bit stays ON for 5 cycles (and reports a false "dead PLC").

If it's changing, you know that PLC is alive and well (i.e. in RUN mode), so if it hasn't changed for 5 seconds, you know it's not.

Use a Delta contact with inline NOT contact driving a TMR 5.000s.  If the .Done bit turns ON, that PLC has not reported a time change in 5 seconds.

STRDLT PL0:SD
NOT
TMR T0 5000

A side effect of this specific mechanism is that everyone can see the status of the Real Time Clock in all the other PLCs, if needed!

[brucek]

Thank you that is a great idea.