Many SCADA (Supervisory Control & Data Acquisition) systems with HMI (Human-Machine-Interface) computers or OI’s (Operator Interfaces) use PLC’s (Programmable Logic Controllers) as the “brains” for monitoring and control. ILADY (I Like Acronyms, Don’t you?)
When communication is lost to a local PLC, alarms and question marks, instead of values, will typically appear on the screen, so the operator is aware that a major problem has occurred. But what happens if a local or remote PLC is not running its logic, such as, if it faults or is put into program mode? What if a remote PLC is not communicating?
Unless the SCADA system has been designed to monitor the complete status of all PLC’s, an operator may be watchng a screen, yet see no indication that an entire system has stopped functioning, or has entered a dangerous condition.
Communication to a PLC is not usually interrupted when it has stopped running its logic, as long as it still has power. That’s a good thing. If it is faulted, you will need communciation to see what the problem is. If it is being programmed, you may not want it to be running logic, but you will still neeed to “talk” to it.
The potentially bad thing is that a PLC that is not running will hold all its data table values in their last state, prior to faulting or being put in program mode. Since communication is unaffected, everything can appear to be normal on the operator’s screen. Motors shown as running may be stopped in reality. Anything may be in a different state than what is shown. The operator may eventually notice that flows, or temperatures, or other analog values are not changing, and trends are flat lines.
A good system will monitor PLC status file bits or words, monitor digital outputs for a change of state, use PLC-to-PLC “heartbeats,” or use some other method to monitor PLC run, as well as communication status. Unfortunately these days, this may have to involve “remote macros” or “derived tags,” but there should be a way to do it. Critical alarms can then be added to alert the operator in a timely manner.
A PLC’s memory-backup battery status should be monitored as well, since it may lose its program and have no logic to run if power to the PLC is lost for more than a few minutes.
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