Fast Alarms Between PLC and HMI


Thread Starter


How can we make an hmi operator panel detect fast alarms? The scan cycle of the hmi is at least 100 ms, while the scan cycle of the plc is between 10 and 20 ms. So any alarm happening in less than 100 ms will not be detected by the hmi. We have thousands of alarm messages, many of them are fast…

Arlen Jacobs

I would recommend that you create plc logic to detect the alarm, then hold a tag high for the HMI to read it. I would recommend that the PLC hold it high for at least 1 second, but your timing, network & HMI speeds will have to be tested out.

The problem you have is Nyquist frequency. If your HMI is only sampling once every 100ms, then the PLC must hold that tag high a minimum of 200ms to ensure the HMI sees the tag value change.

100ms update rate is fast for an HMI, and just because you set the HMI to 100ms it does not mean that you are actually getting updates every 100ms. There are several real world factors that can slow down the update rate that you actually see at the HMI.

You'll have to play around with it, but I would recommend starting with the PLC holding the alarm tags high for 1 second. Set the HMI to scan every 500ms (which will free up some network, PLC, and HMI bandwidth).

Jeremy Pollard

Use an alarming sbr in the PLC to time expand.... with Rockwell it would be the TT (timer timing Bit) and set the timer for the length of time you need.

Cheers from: Jeremy Pollard, CET The Caring Canuckian!
Crisis, necessity, change


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PLC Programmer

This is an easy One.

You can latch the alarm signal at the PLC ladder logic and add another DI signal from the HMI to reset / unlatch this signal, given that the triggering condition does not exist.

So the trigger signal from the PLC stays On as long as you want until the condition persisting is corrected and alarm reset (This is for critical alarms).

Further if You just need to see a notification on the HMI.
You can latch the alarm trigger signal to be send to the HMI and unlatch it with a timer delay of some time over 100ms in the PLC Ladder logic. (Or just use an Off delay timer of time greater than 100ms). Such alarms do not need a reset, given the alarm triggering condition has been cleared.

Hope that clears your doubt.