AB PLC5 Scan Rate "Spikes"

bhyde

Member
Join Date
Jun 2007
Location
Benicia, CA
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I’ve run into a strange situation that I’m struggling to explain. We have several AB-PLC5’s that have scan rates that “spikes” up for short periods of time. This usually causes the processor to fault because of a watchdog error (yes I know I can increase the watchdog value). The “spike” is typically about 800ms to 1200ms and the normal scan time is around 50ms. When the programs are fully loaded, we might see a normal scan rate of about 100ms.

The programs do contain several For/Next loops in high program numbers, but even if I force all the loops to execute on a single scan, I only see an increase of about 300ms max in the scan rate. In actual running conditions only one or two loops ever run on a single scan, so I’m not convinced the For/Next loops are the problem.

It’s a fairly typical PLC5 setup with RIO, Ethernet, and a couple of remote racks. There is also some HMI’s, a batch server, and a data logging system.

So the question is: Has anyone seen big jumps in scan rates that are not related to for/next loops? Maybe a communications problem or something related to I/O?
 
Does your program use an STI (selectable timed interrupt)?

If an STI is set too fast it can cause a problem with overall scan time. Apart from the FOR NEXT loop that can cause a scan time slowdown, there could also be an issue with Jump instructions (JMP JSR). Nested jumps and especially when you jump backwards can be tricky. AB does not recommend jumping backwards.
 
If you are using a new platform PLC5, are using the STI AND have BTR instructions in the STI, your problem may be communication related. The processor will sit in the STI waiting for a BTR programmed in the STI to finish. If that is getting hung up for some reason you may get a watchdog fault. I think the same thing applies to immediate input instructions.

Keith
 
There are no STI's or JMP instructions, but there are many JSR's. Most of the JSR's call routines with lower program numbers (i.e. ladder 65 calls SBR 10).
What is strange is that when I run on a test PLC with no I/O or remote communications, I don't see the problem. It is only on PLC's with I/O and other loads that the scan rate has a problem. Why would a PLC with I/O or communications scan any differnetly than one without. My understanding was that comm and I/O are handled during the "Housekeeping" portion of the scan.
Is it possible the "housekeeping" is running long?
 
bhyde said:
There are no STI's or JMP instructions, but there are many JSR's. Most of the JSR's call routines with lower program numbers (i.e. ladder 65 calls SBR 10).
What is strange is that when I run on a test PLC with no I/O or remote communications, I don't see the problem. It is only on PLC's with I/O and other loads that the scan rate has a problem. Why would a PLC with I/O or communications scan any differnetly than one without. My understanding was that comm and I/O are handled during the "Housekeeping" portion of the scan.
Is it possible the "housekeeping" is running long?

I had something similar happen but on Controllogix. During In-house testing (without actual IO connected we had very fast scan times but when we made it to the real world with all IO connected via controlnet our scan time blew-out dramatically. We traced the problem down to an overworked controlnet network and we ended up running two networks, one for IO and the other provided a path form the ethernet modules thru to cpu for the scada system.
 

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