Nikolay

How 'atomic' is PLC operation in regards to input from HMI?

3 posts in this topic

Dear Community,

I have the Allen-Bradley 1769-L32E Compact Logix 5332E PLC and a C-more touchscreen.

Assume that there is a pushbutton at the touchscreen (set up as momentary) that sets a bit at particular PLC address (call it hmi_button_1)

On the PLC this bit is checked twice within one routine - on two rungs that are directly adjacent to each other.

 

plc_atomic.png.3bce83eb273d61685e5f19025

 

So, my question is:

Is it possible that the state of hmi_button_1 will be different within one routine  execution cycle? For example rung 1 reads it as FALSE while rung 2 reads it as TRUE (or vise versa)?

Or state of bits is guaranteed to be the same for one complete routine execution cycle?

 

Best, Nikolay.

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since this is a  "XIC" and use in the ladder as input condition , it will be same in one scan cycle 

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I don't know if the communication from the HMI to the PLC is synchronous with the scan. It may be asynchronous as the I/O evaluation. If you are really concerned then copy the state of the input bit to a bit parameter near the start of the scan and use that bit throughout your code.

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