Electromechanical Cam timer replacement

I have a small project where I need to replace an electromechanical cam timer (something similar to these: https://micronor.com/products/cam-timers/) with a PLC.

However, key to retaining similar functionality to the current system, I need the output contacts from the PLC to remain in their previously set positions throughout a power loss/ PSU or CPU failure scenario. Any thoughts how this could be done?

I was thinking using separate latched relays with Set and Reset coils for each output, but this has some sacrifices in terms of space and then having a power-up or First Scan bit to allow realigning the PLC code with the desired cam position. It would be acceptable to have operator input through an HMI to align the PLC mode/ step or time with the desired set before resetting via another input.

Hardware is likely to be based on an A-B Micro8## or MicroLogix platform, though other options exist.

Any other smarter solutions?

A possibility that comes to mind is using a Siemens S7-1200. Make a data block for your virtual cam positions and check retain on all important tags. My understanding is a capacitor supplies power to write these values to flash memory on power down. Nothing is infallable and there’s a bit available to alert the program if retained memory is lost but I haven’t seen that happen in my limited experience with these controllers.

an output sequencer most plc's have that function built in
use something like the AB Micro800 series

2stepsforward said:

... I need the output contacts from the PLC to remain in their previously set positions throughout a power loss/ PSU or CPU failure scenario...

Click to expand...

Not sure about AB. In other PLC's, this is a common function. You set the outputs that you want remembered to be "Retentive".

For the cam timer, in Automation Direct products you would use an instruction called DRUM.

Thanks for the thoughts. The sequence output function looks a smart way of minimising the code that needs writing.

I'm aware of retentive tags on some/ most PLC platforms, however since my final outputs are externally powered via UPSs I really want my PLC outputs to remain as-is even on a PLC power supply failure. Retentive tags might let me repower up the PLC but I can't see a way they would allow a fail last operation during a CPU or PSU fault. Am I missing something? How would the output contact/ source linked to a retentive tag stay powered/ closed during a PLC power loss?

Battery backed memory - HR (holding relays) in Omron for example. When the PLC powers up again the outputs are on after the first scan and stay on till turned off in the program. If the program turns them off after the first scan they turn off.

Since you already have UPS power, you could power a 24VDC PLC from the same UPS. Redundant 24VDC power suplies are readily available to alleviate concern of a PSU fault. I would use a high quality PLC and trust it not to fail. It sounds like the program will be simple enough that with proper development there should be no risk of program errors causing faults. If you want contacts retained on a PLC problem, external latching relays will likely be used.

If retained contacts are that important, maybe a cam system is still viable. Possibly a cam assembly could be servo driven with position feedback to a PLC. Just an idea to throw out there...