Siemens Power Management System Lockup

[SBMerrick]

Our facility has a power management system based on a Siemens S7 PLC polling about 100 Siemens 9610 multifunction relays which are daisy-chained together with 4-wire RS485 serial cable. We recently experienced a total loss of facility power, and the power monitoring system completely froze up for at least two minutes right when we needed it most. We have gotten very little support from the vendor in resolving this issue, so I am hoping that some of you talented individuals on this forum can help point me in the right direction. My present assumption is that the most likely answer is that simultaneous alarming of over 100 remote terminal units attempting to alert the central PLC over daisy-chained serial cable caused a communications bottleneck which lead to a system hangup. Does this seem reasonable, or am I chasing a red herring? Any thoughts would be greatly appreciated.

Steve

 

[Noway2]

That is one distinct possibility. RS-485 allows for one transmitter at a time. Depending on the protocol used, it may or may not have built in arbitration to prevent all devices from transmitting.

Another possibility to consider is if the power glitch caused the PLC to lockup.

Someone with more experience than I in siemens PLCs might be of more help.

 

[jraef]

Is there a UPS on the PLC? If not, how would you expect it to do anything anyway? Assuming there was one, the PLC typically will not "lock up" because of a flood of information, but the flood will slow down the message response time if the network is configured incorrectly. For instance, if whomever set up that network configured every one of the meters as a Modbus RTU with direct transmit capabilities (as opposed to being polled from the PLC acting as a master), then if they were all communicating simultaneously, the PLC CPU would be dealing with them in a FIFO stack and the aggregate response time would be out there.

I would start with the integrator or whomever built the system. If that was Siemens and it was within the last 10 years, call the Automation Technical Support line at 1-800-333-7421, they have lots of really good people there. If it is older than 10 years, you may need to pay for support depending on what you have.

 

[SBMerrick]

Thanks for the responses so far.

jraef: Yes, both the PLC and the RTUs are provided with UPS power. It is a very recent installation, but we are having difficulty getting the vendor to acknowledge that there is a problem. I will try the support line you suggested and see if Siemens has any ideas.

 

[jraef]

Just be aware that if Siemens didn't build the system, they may not be able to help you much with design and configuration problems. Good luck.