What happens if the PLC is turned off?

Not a PLC. but years back I had a smart VCR that after a power failure would remember the channel scan settings, the nicknames for each channel, date & time and if something was programmed to record over the next 14 days.


The problem was if the power glitched for a half second it was a total wipe. If a gust of wind went by our house the power would cut off momentarily about 45 seconds later, so I figured a wire was broken internally about 1/2 mile northeast of us.


I ended up putting the power through a 30 second timer, if there was a glitch the VCR would be off 30 seconds.



I also program in similar timers when a PLC is controlling the power to a VFD or servo controller.

parky said:

Seems to be that many companies in the States wire the PLC power via the Estop circuit,

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They still do this *****! (Broad brush, I know)

We have a customer that bought 7 Uniloy Blow Moulders and Trimmers last year. Every single one had the external equipment power through the E-Stop.

If you hit the E-Stop on the BM, 300 metres of conveyor after it stopped!

Good money for us to go in and put it right but in the name of all thats holy WHY?

Any respectable PLC brand should keep both program and retentive data at a power-out.
That said, brown-outs and very short power cycles can create havoc, and I dont think any PLC brand actually promises to handle either of these.
I have installed voltage monitoring relays at certain projects where the sites were notorious for brown-outs. The voltage monitoring relays would then handle that power is shut down and powered up in a proper manner.

In my experience, modern PLCs and controllers are very tolerant to power losses and should not lose program and data just because power was lost - this is a part of industrial-grade design after all.

Still, everything that was said above holds true and it is definitely not a good idea to power down any machine "brains" on e-stop. I believe designs like that are legacies from some distant past when any electronic device was considered not safe enough to rely upon and design philosophy "kill the machine, screw everything" prevailed.

Having said that, the OP mentioned "cold restart". I think this is a term used in some IEC 61131-based systems (Codesys, Multiprog and their derivatives) for a special type of restart when it is deliberately required to reset all the retentive data. Other PLCs have special procedures for that, although they are usually not called "cold start".

Of course, just cycling power should never wipe any retentive data - or there is something seriously wrong with the hardware.

My final check on commissioning a PLC system is to a power cycle to ensure that the system recovers correctly after a power failure.

If its a modern Siemens system, which has been programmed so that it can restart after a power cycle, then it wont care about it. We power cycled the things multiple times a day on our test rigs and nothing happens.

If they didnt bother to set retentive memory properly, all bets are off. You might have to manually reset an entire line to fix it, and recalibrate everything. Even retune all the PIDs and timings if noone copied them down.

The only time you trust a PLC to restart is when the guy programming it will happily flip the breakers five minutes before they need to leave to catch a plane home. It will work or its their problem.