Addressing issue

I'm trying to write a program to better understand the various uses for a LIM Instruction. I'm having a issue with it reading the Counter value. I would greatly appreciate the help.

see if this helps ... if not, consider posting a screen shot of your setup ... we'll be glad to take a look ...

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Jose,

The LIM instruction is like two comparison instructions combined (ANDed) together. There are two major variations:

1. NORMAL CASE: When the Low Limit value is less than High Limit:
IF (TEST >= LOW LIMIT) AND (TEST <= HIGH LIMIT), THEN OUTPUT = TRUE
(Notice that when Test = Low or High Limit, ouput is TRUE).

2. ABNORMAL CASE: When the Low Limit value is entered as greater than or equal to the High Limit:
IF (TEST > LOW LIMIT) AND (TEST < HIGH LIMIT), THEN OUTPUT = FALSE
(Notice that when Test = Low or High Limit, ouput is TRUE).

In both cases, if the test value is outside the comparison limits, then the output is the opposite of those shown above.

Thank you that did help a lot.

Lancie1 said:

Jose,
2. ABNORMAL CASE: When the Low Limit value is entered as greater than or equal to the High Limit:
IF (TEST > LOW LIMIT) AND (TEST < HIGH LIMIT), THEN OUTPUT = FALSE
(Notice that when Test = Low or High Limit, ouput is TRUE).
...

Click to expand...

There is nothing "abnormal" in such case.
This is a common case on machines with rotary encoder.
And such behavior of the zone comparison instruction is one of few AB advantages.
In Omron, the emulation of this requires 6 branched in-line comparison instructions.

Greetings Jose ...

regarding your statement:

I'm trying to write a program to better understand the various uses for a LIM Instruction.

Click to expand...

based on that, you might be interested in the link posted below ...

the idea is that many programmers try to use the LIM (Limit Test) instruction to KEEP/MAINTAIN/HOLD a variable within a certain RANGE of values ...

for example, suppose that we tell the operator: "Don't ever set this dial any LOWER than 20 – and don't ever set it any HIGHER than 80" ... some operators will naturally start cranking the dial too low – and too high – just as soon as we walk away ...

at first glance, the LIM instruction seems to be a natural programming choice for keeping the variable within an acceptable range ... the problem is that the LIM only tests to see whether we're INSIDE or OUTSIDE of the range – but it isn't all that handy when it comes to actually "fixing" an out-of-range problem ...

specifically, the LEFT end of the rung is pretty easy to set up ... but then ... what do you do with the RIGHT end of the rung? ...

the picture in the following post is a very common way of "limiting" a value and keeping it from getting out of line ...

http://www.plctalk.net/qanda/showthread.php?postid=23689#post23689

good luck with your studies ...

Sergei Troizky said:

There is nothing "abnormal" in such case. This is a common case on machines with rotary encoder.

Click to expand...

Sergei, Okay, you convinced me. Perhaps abnormal was a poor choice of words. Unusual would be better.

You have to admit that a control case where Low Limit = Largest Number, and High Limit = Smallest Number is at least unusual, if not completely odd.

Tank Level? No, the highest number will be the fullest (high limit).

Motor Speed? No, the highest speed will be the high limit.

Travel Distance? No, the longest distance will be the high limit?

You can see what I mean if you think about it. Can you think of any other cases except the rotary encoder? I am not sure how a LIM instuction would work there, because the encoder continually goes from high to low and back.