Safety PLCs - PLCS.net

lanczos · September 18th, 2023, 11:50 AM

Do all of the so-called "Safety PLCs" have a software-generated watchdog or heartbeat, so that if the software locks-up, crashes, or enters an infinite loop (for whatever reason), a hardware layer will detect the loss of the heartbeat and move all outputs to a safe state? Is that a standard feature?

Thanks!

mk42 · September 18th, 2023, 12:03 PM

For every safety system that I've used, it's there somewhere, yes. Otherwise it wouldn't be very safe.

In Siemens systems, the outputs themselves are "smart" and do the heartbeat detection as part of the internal diagnostics. If they find a fault in themselves, or if they lose comms to the PLC, then they go to the predefined safe state.

Steve Bailey · September 18th, 2023, 12:08 PM

That's a feature of every PLC I've ever dealt with, not limited to safety PLCs.

lanczos · September 18th, 2023, 01:15 PM

Thanks, figured as much, just wanted to confirm.

September 18th, 2023, 11:50 AM	#1
lanczos Member Join Date: Sep 2023 Location: New Mexico Posts: 2	Safety PLCs Do all of the so-called "Safety PLCs" have a software-generated watchdog or heartbeat, so that if the software locks-up, crashes, or enters an infinite loop (for whatever reason), a hardware layer will detect the loss of the heartbeat and move all outputs to a safe state? Is that a standard feature? Thanks!

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September 18th, 2023, 12:03 PM	#2
mk42 Lifetime Supporting Member Join Date: Jun 2013 Location: MI Posts: 3,151	For every safety system that I've used, it's there somewhere, yes. Otherwise it wouldn't be very safe. In Siemens systems, the outputs themselves are "smart" and do the heartbeat detection as part of the internal diagnostics. If they find a fault in themselves, or if they lose comms to the PLC, then they go to the predefined safe state.

September 18th, 2023, 12:08 PM	#3
Steve Bailey Lifetime Supporting Member + Moderator Join Date: Apr 2002 Location: The boondocks of Western Massachusetts USA Posts: 8,355	That's a feature of every PLC I've ever dealt with, not limited to safety PLCs.

September 18th, 2023, 01:15 PM	#4
lanczos Member Join Date: Sep 2023 Location: New Mexico Posts: 2	Thanks, figured as much, just wanted to confirm.

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