saruans

fx3g plc pls bit

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hello,

I have problem with fx3g series PLC. I attached ladder program fragment.When I set momentary bit M1 don't send PLS M4, but if i set momentary M2 program running well. Why is it?

PROBLEM.jpg

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Actually M4 is pulsed high, but then in the second rung (M2) it is set low again since M2 is not high at the same time. If you want to have two ways to trigger M4 you must use two temporary bits (e.g. M10 and M11 or something else free). M1 triggers PLS M10 and M2 triggers PLS M11. Then after these two insert a ladder with M10 and M11 in parallel that triggers M4.

The technical point is that between PLS M4 and PLS M4 (between the rungs M1 and M2) the bit M4 is actually high. But it will never remain the same state throughout the scan unless M1 and M2 are triggered at the same time. Since PLC's are looped the bit will remain its state until a new state is detected and in this example M4 is high in the first line, but then in the second line M4 evaluates to FALSE hence it goes off again instead of remaining on the complete round-trip.

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No problem. Just remember to either use temporary variables when you are triggering the same outputs. Alternatively use SET/RST functions. But the last line of code in a sequence will always win if you don't use SET/RST, or use dedicated (temporary) variables.

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If you use 'check program' it will give you a double coil error.

Because it's an error to use the same output twice - as you have now found out. :)

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You can put M1 and M2 in parallel (OR) to PLS M4 and after that M1 MOV K20 D1 and M2 MOV K30 D1 on separate lines. In this way you will remove the double coil error and It will work in the same way.

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