Hi, I'm an old reader but new joiner of this community.

Right now I'm actually facing this problem: the plant where I'm now working at has an Advant PLC that's controlling part of the motors, with program running at 250 ms. This PLC is modbus interfaced to a DCS, whose control module scan time is now 250 ms too, but as the customer has filled it with several logics, now the available processor idle time is below alert levels, making difficult any on line download to it.

My idea would be to raise DCS control modules' scan time up to 500ms, but customer fears problems with communication and possible shut downs.

What's your opinion about that?

Thanks in advance

 

It should be possible to create fast and slow scans from the DCS system to reduce unnecessary scans and hence reduce load on PLC. Presumably you control writes and reads to the modbus variables in a controlled sequence?

Simon

 

What sort of Advant PLC? 70/110/200/410/450 ?

250 ms for motors is too fast and likely that's where a reasonable amount of your CPU time is being wasted. You *might* be able to easily change the scan time of the CONTRM that contains the Motors, but without knowing your PC program structure then I cant tell you for sure. half or 1 second scan time is usually sufficient for a Motor block.

250ms is not fast for a PLC link - your message rate is nowhere near that fast. The message rate of a MODBUS plc link is actually controlled by the SENDREQ elements which will ONLY process 1 message at a time. It is unlikely that your modbus link is producing a significant CPU load.

If you have an AC400 series controller then you need to run an ANPER and collect a detailed breakdown of which tasks are using CPU time. For example, improperly configured TTD logs can use up too much CPU, excessive subscriptions or subscription rates from the HMI can also be a problem. Unresolved Tags on an advant station also cause high CPU load in controllers with MB300 networks. ANPER will help you see what is using the CPU load. It may not be PC programs at all.

Rob
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