SLC 5/04 User Fault Routine

IsaacC

Member
Join Date
Jul 2010
Location
Minnesota
Posts
4
Hi all,

So a SLC 5/04 went down with a fault but ended up causing some trouble for other systems so they want a way to notify these other PLCs. I've set up a fault routine but I'm not sure how certain instructions work with this routine.

Specifically, do I need to OTL a bit to keep it high? Or, since it only scans the routine once and then halts, will an OTE keep a bit high? The bit is used elsewhere in the program without OTU's so I wanted to avoid an OTL if possible.

Thanks!
 
In the fault routine turn on a unique bit. Doesn't matter if it is OTE or OTL because as you mentioned, it will only ever scan the logic when a fault occurs.

You also have to clear the fault. When the fault routine finishes it will return to the normal logic and resume where it left off.

I personally prefer to put the Messaging logic outside of the fault routine and trigger it based on that unique bit. At some point you will want to reset that bit and that would typically be done with an OTL. That bit can be examined (input) elsewhere in the logic but it should not be controlled (output) elsewhere other than the OTU.

You can write logic as well to say that once the message has been completed (done bit), do not allow the fault to be cleared anymore and let the processor fault. But that is all in how you want it to work.

If you trigger the message inside the fault routine and do not clear the fault, the message will not get serviced and will not send as the servicing of comms happens at the end of the program scan (end of LAD 2).

You could even not clear the fault and just turn on the bit. You could configure the other controllers (assuming other PLCs) to read that bit. If the bit is on, they know you have faulted, if it is off then no fault.

OG
 
Last edited:
Ah, that makes sense. I'm not worried about clearing the fault, just what happens before it halts.

The problem I'm running into is the other PLC's are reading from this one and it services requests like nothing is wrong. I just wanted to set a bit to let the other PLC's know all is not well.
 

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