Siemens 317T PLC power off/on

I am looking to detect when the power to the PLC has been cycled but not when it has simply been set to STOP and RUN again.

Have you looked into the cold restart OB (OB102)?

http://www.infoplc.net/files/descargas/siemens/infoplc_net_sitrain_12_obs.pdf

I'm fairly certain only warm restarted is permitted for 300 series Siemens PLC's.

I may be wrong though

As far as I know, the 300 PLCs treat a power cycle and Stop->Run transition the same in the program. I'm not sure how it would look in the diagnostic buffer.

However, you could potentially look at the behavior of the devices external to the PLC. Something like a managed Ethernet switch will usually have a longer boot up time than the PLC will. If they are controlled by the same circuit and the PLC is plugged directly into the switch, you could do a test in OB100 to see if the port on the PLC is active or not.

In a run->stop->Run transition, the ethernet connection will never be lost.

In a power cycle, the port might still be waiting for the partner to come online.

You'd have to test the power on behavior of your system to see which components to monitor for your test.

I'm fairly certain only warm restarted is permitted for 300 series Siemens PLC's.

Click to expand...

You are right, the 300 series only has warm restart.

The only possibility I can think of if it's very important to you is to have the CPU connected to a UPS with a digital signal tell you when power is off.

What do you mean by power cycle - example of what exactly you wish to detect and why, I think would help find a solution.

The cyclic overrun of encoders on the system is lost when the power to the CPU is lost. Therefore I want run a routine only when the power is lost and regained

Look in the tempstack of OB100

I think the variable #OB100_STRTUP is what you looking for

Other possible values·
B#16#81:Manual warm restart·Switch/Pg
B#16#82:Automatic warm restart. Power failure
B#16#83:Request for manual hot restart
B#16#84:Request for automatic hot restart
B#16#85:Request for manual cold restart
B#16#86:Request for automatic cold restart

Sounds very promising.

You don't know by any chance if the bit is set even if the plc is: put into stop, powered down, powered up, put into start?

I don't have a plc to hand for testing right now

Thanks

Its in ob100 and its a byte not a bit. OB100 is only executed once just before the PLC goes into run after power on.

I have used this in the passed to check if the run/stop switch was used to trigger the stop and that worked out fine.