I am after an opinion and reasoning on the following situation.
I have a water tank (T01) that has a high level switch (digital) connected to an AB CompactLogix PLC (PLC1).
The feed pump control for the this tank has been moved to a different AB CompactLogix PLC (PLC2).
Both PLC’s are connected to Ignition 7.9.
My question: Is it better practice to use the “Value Change” script of the Tank High Level switch (PLC1) to write to a Pump Inhibit tag in PLC2 or use messaging between the two PLC’s?
Communication between the 2 PLCs is light and messaging allows for “Maintenance Crew” to fault find without the need to access scripting in Ignition. Both ways will work, but I am wondering what the experts think and why.
Use Producer/Consumer tags directly between the two processors. Include a constantly changing field in the produced tag (from PLC1) to act as a heartbeat when consumed at PLC2.
You absolutely cannot trust a SCADA using request/response protocols in such a control loop.
Messaging initiated on PLC2 to read from PLC1 would be my second choice.
I appreciate your prompt reply and for sharing your knowledge. I had thought about using the “Tag Quality” function in Ignition as a “Heartbeat check”, but I think communication directly between both PLC’s is the better option.
Again thanks for your reply and thoughts.