RS Logix Compare (CMP) instruction syntax

dginbuffalo said:

Oops- should have been:

VarA>VarB or VarC or VarD

This should do the trick if he wants to know if VarA is larger than the other 3 tags i would think.

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Be very careful with that one. It won't do a Boolean compare. It will bitwise OR VarB, VarC, and VarD into a single result and them compare if VarA is greater than that result. See AB knowledgebase technote 52939.

If Var B=5, Var B=4 and VarC=3 then they will be bitwise OR-ed and the result will be 7. If VarA=6 then the test will fail. VarA is greater than all the others, but it is not greater than the bitwise OR-ed result of 7, so the comparison will be incorrect.

It is better to use three comparisons in series. This way you are sure of exactly what you are instructing the processor to do. Ambiguity in software is not worth it when expensive processes, safety, plant uptime, and 3:00 am phone calls from Bubba are on the line.

TConnolly said:

Be very careful with that one. It won't do a Boolean compare. It will bitwise OR VarB, VarC, and VarD into a single result and them compare if VarA is greater than that result. See AB knowledgebase technote 52939.

If Var B=5, Var B=4 and VarC=3 then they will be bitwise OR-ed and the result will be 7. If VarA=6 then the test will fail. VarA is greater than all the others, but it is not greater than the bitwise OR-ed result of 7, so the comparison will be incorrect.

It is better to use three comparisons in series. This way you are sure of exactly what you are instructing the processor to do. Ambiguity in software is not worth it when expensive processes, safety, plant uptime, and 3:00 am phone calls from Bubba are on the line.

Click to expand...

Thanks for the heads up. I can't say I've ever used this but it seems like it would work but now I'm slightly confused from the technote! Here it is for reference.

There isn't anything to be confused about.

OR, AND, XOR, and NOT cannot be be used as comparison operators in a CMP expression, but they may be used as expression operators.

Put another way, you may use only =, >, <, >=, <=, and <> for the comparison. Everything else will evaluate as an expression to produce a result against which the comparison is made.

As I said before though, it is better to use three comparison instructions (in this case GRT instructions) in series so that there in absolutely no ambiguity in what you are instructing the processor to do rather than guess how the compiler will interpret the expression.

For anyone finding this in 2021, I found this thread misleading. At least in version 33.00.00, it is definitely possible to do multiple expressions in a CMP instruction! The operators you're looking for are '&&' and '||'.

Here's an example of a working CMP expression:
((SomeVar > LoLimit) && (SomeVar < HiLimit)) || IgnoreLimits

charling said:

For anyone finding this in 2021, I found this thread misleading. At least in version 33.00.00, it is definitely possible to do multiple expressions in a CMP instruction! The operators you're looking for are '&&' and '||'.

Here's an example of a working CMP expression:
((SomeVar > LoLimit) && (SomeVar < HiLimit)) || IgnoreLimits

Click to expand...

Not available in V30, the latest version I can check.

Does not work in v32 "CMP, Operand 0: Logical operators (!, &&, ||, ^^) are not supported."