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Outputs after power failure

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  • MVP 2023

Perhaps the only problem is programmed in PLC logic of work.

If we have a cyclic process, then it's easy to go back to the previous state before power is turned off.

However, if the power supply turning off can cause uncontrolled events in the system, it is impossible to return

to the previous state. It usually causes a complete restart of the system or the operator's intervention needed.



I liked the mechanical "PLCs" installed in old washing machines. Turning off the power did not affect the cycle of work.

After turning on the power, all processes continued until the end of the program.

An electric motor and a drum with contacts are good "memory registers" that are not erased when switched off. :)

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  • MVP 2023

Kratmel's love for his washing machine mechanical timer is easily duplicated in plcs, using my oft quoted (but very unfairly consigned to the garbage bin/mostly ignored) method of using a counter and compares.  It's a bit like the standard drum setup, except that you are essentially making your own, and it's not hard to do at all.  A map (like the operating sequence illustration that used to accompany such mechanical timers) is invaluable, and I almost always do this in an excel format.  I have also considered using a multi-channel sound editor, which in theory might actually be easier to achieve the desired results of being able to adjust things easily as necessary.

The count is the only thing that needs to be stored into an MI, or ML.  The rest of the actions, that are all based on compares, will remain exactly the same on power up, or restart from pausing the count where the pause also initiates a total turn off of every output being controlled.  The only proviso might be the need for delayed startup/load shed in some instances on high draw outputs through a short timer linked to the count.  All involved outputs are set to power up in off state and the subsequent scan adjusts everything to suit.

cheers, Aus

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