Schneider Electric PLC M221 - Detect changes from STOP to START mode on ladder

[Paulo Balbino]

Hello,

I am trying to detect when the PLC changes from STOP to START mode. This can be considered an edge case scenario, but I would to analyze all potential problems on this approach.

Hardware: PLC Schneider Electric M221CE24R
Programming software: Ecostruxure Machine Expert Basic

Current scenario:

Assuming I have configured the "Fallback Behavior" to "Fallback values"

When the mode from PLC is changed to STOP mode (by manually switching the mode button of the PLC to STOP), all the (Q) outputs are disabled

Then when changed back to RUN it will set everything back to the previous state.



Assuming I have configured the "Fallback Behavior" to "Maintain values"

When the mode switch from PLC is changed to STOP mode, the (Q) outputs stays the same

Then when changed back to RUN it will continue processing the ladder normally



What problem I am trying to solve:

On this project, activation of outputs/relays are related to turning on high power systems. So there must be a delay between enabling each of the outputs (which is implemented on the ladder).

If the PLC during operation is switched to STOP mode, if configured to "Fallback values" when put to RUN again it will simply enable all the outputs (which will enable all the high power systems at the same time)

In case of "Maintain values" it will keep the values during STOP which is ok for some of these outputs (related to high power system activation) but for other parts of the operation for example a motor which is operating since the ladder is not executing no sensor (collision sensor/switches) input will prevent/stop the motor.

If I can have a way to detect this scenario (STOP to RUN mode) on ladder I can handle this case.

Another question: Should I care about this case? This case will only be a concern when someone physically opens the control panel and set the PLC to STOP mode. My main concern is safety.

Thanks a lot
Paulo

 

[parky]

Most PLC's have a first scan bit that is on for the first scan i.e. from stop to run, this also applies if powered down then back up, so you could use this to do some logic tayloring it to your needs.
I believe it is system bit %S0 is first scan bit.

 

[drbitboy]

is there no "First Scan" bit in this PLC?

If not, could this be used to make a bespoke first scan bit?

... but for other parts of the operation for example a motor which is operating since the ladder is not executing no sensor (collision sensor/switches) input will prevent/stop the motor. ...

Relying on the PLC with no PLC-external physical collision logic ("hard stop") override sounds like a design problem to me

 

[drbitboy]

Most PLC's have a first scan bit that is on for the first scan i.e. from stop to run, this also applies if powered down then back up, so you could use this to do some logic tayloring it to your needs.
I believe it is system bit %S0 is first scan bit.

+1.

There is also a warm start bit: %S1.

There is also the %S13 bit which is set to 1 in the first cycle after the controller was started.

See here, bottom of the page.

 

[Paulo Balbino]

Tks, exactly what I was looking for