Q Series Scanning Order and IO Refresh

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Hello everyone,

Long time listener first time caller :)

Relatively new to Mitsubishi PLC's, and I have a general question on the scanning and IO refresh for Q series PLC's. I haven't been able to find the answer searching so if I missed it, then, my bad.

For PLC's that have multiple programs running in them, at what point is the IO actually updated? For example:

You have two programs running. A and B. For both programs the "Program Parameter", lists A before B, and both are set to "Scan". (I am assuming this means scan them in series as fast as possible.)

You are controlling Y1, and you set the output to ON in Program A. Then in Program B you set Y1 to OFF.   What happens?

Does Y1 get switched ON immediately when its rung gets scanned?

Does it get switched ON at the end of Program A's scan?

Or does it never actually switch ON, because the IO is updated after ALL programs are scanned, and it gets set to OFF in program B?


Also, does it function the same with digital registers? What about MELSEC registers?


Hopefully my question makes sense.

Thanks for any help, and if you can point me to a manual that answers this question that would be great too!


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inputs and outputs are refreshed at the end of the sequence. This means if program B resets Y0. Y0 will never turn on,

If you would like to set an output directly use DY0 then it will be activated immediately.

All other devices are changed directly 


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