LogixDesigner 5000 - Monitoring of powerflow through a CMP instruction

SilverShotBee

Member
Join Date
May 2011
Location
England
Posts
46
I've recently picked up AB again after spending 5 years on Siemens and noticed a frustration immediately with monitoring of CMP (Compare) instructions

How do I know that the CMP instructions are outputting a logical 1 or True? Am I missing something really obvious here or is there a setting buried away somewhere that switches the flow on between instructions?

Screenshot 2021-12-07 004659.jpg
 
Hey SilverShotBee,
I'm not sure if this will work for you, but I like to use the EQU, GRT, GEQ etc. instructions. This way you can see the actual values being compared.

I hope this helps.
Woolie
 
Hey SilverShotBee,
I'm not sure if this will work for you, but I like to use the EQU, GRT, GEQ etc. instructions. This way you can see the actual values being compared.

I hope this helps.
Woolie

Thanks, I dunno why I forgot they existed, especially as I got the CMP instruction from the compare tab where all the others are! I've been doing a lot of S7-300's recently and CMP comes naturally. I'll ditch that in AB! It still seems a bit ridiculous that you can't monitor those instructions though!
 
Not that it completely justifies it, but in a CMP instruction you can do much more complicated things than just < or >. You can put math on one or both sides of the operator, and use boolean instructions like AND or XOR. So it's not as straightforward as just looking to see the value of two tags.

FWIW, you can hover over each tag in a CMP instruction and it'll tell you the value, so in a pinch you can at least monitor tag values and determine for yourself whether the instruction is evaluating true or false.
 
FWIW, you can hover over each tag in a CMP instruction and it'll tell you the value, so in a pinch you can at least monitor tag values and determine for yourself whether the instruction is evaluating true or false.

I thought that was the case, but in an Add-on Instruction it simply says the variable is unknown (even though it works fine). I'm aware more complicated use-cases exist for CMP, but I don't ever remember not being able to easily see if it was outputting or not
 
The CMP allows for the entry of expressions like a CPT (compute) which also does not display the values of any PLC tags the expression contains. For simple compares, the specific individual instructions that have single element sources that allow the software to display the values are the better choice.

CMP.png
 

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