We have 3 Frame 6B with Mark 5 HMI that interface to AB PLC5 and Honeywell experion scada.

Randomly, the experion scada will receive spurious alarms (typically all the same) which also initiate a gas to diesel changeover.

The alarms that get picked up by the experion are over modbus serial and only last 2ms.

Can someone give me information on where the issue is occurring or how to find it.

I suspect it may be a faulty Mk5 card or a cable short somewhere.

Thanks

 

If the Modbus Physical link is RS485 then there may be timing issues that happens intermittently.

In RS485 it is important the Key Up and Key Down timing of RS485 Transmitters do not interfere with each other. To this end make sure the Modbus Slaves do not respond until after the Modbus Host has keyed down.

Also with RS485, make sure the RS485 A B lines have "fail safe biasing" which insures all receivers are not receiving start bits when no RS485 transmitters are active. See the National Semiconductor Application Note 1057 on 10 ways to bullet prook your RS485.
http://bitsavers.trailing-edge.com/pdf/national/_appNotes/AN-1057.pdf