ISPsoft problem

sigmadelta said:

Look at the logic just above Network 34.... you have a coil M203.

This means your M203 will be set at Network 34, but when it reaches Network 33, it will be off, then on again at network 34. If your rung at network 34 becomes false, then the value of M203 will come from Network 33.

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drbitboy said:

Also note that the logic handling the bits [Lswitch] and [M122] at the beginning (left) of Network 34 are rising edge instructions, and so can make the rung true for only one scan at a time, and never be true for two or scans in a row*.


It is likely that the combination of these two rungs will perform other than what is desired. How do want want M203 to behave based on the inputs (Lswitch, M122, Mmode, Llevel, X15)?



Because there is a funny saying in PLC programming: "the only thing worse than a PLC not doing what the programmer wants it to do, is when the PLC does exactly what the programmer tells it to do." Or, to state it another way: the PLC always does what the programmer tells it to do; what the programmer wants the PLC to do is irrelevant.


* Unless, because they are ORed together, exactly one of [Lswitch] and [M122] were 1, and the other 0, and alternating on every other scan, which would be extremely unlikely assuming they are based on discrete inputs.

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