LIM instructions

Can anyone provide me with some insight as to how LIM instructions work. I understand that it looks for a value to be within a certain range; however, I am having trouble understanding why, in this application, the higher value is in the "low limit" area and the lower value is in the "high limit" area of the instruction. If I swap the tag positions and we are "within our window" the timer starts? (see attached photo)...confused...

The limit instruction can work two different ways. When the low limit is a lower value than the high limit the instruction works as you describe. Any value between the two limits will make the instruction true. If the low limit is a higher value than than high limit than it works the opposite way. Any value outside the the limits makes the instruction true. In your case it looks like any temperature outside the accepted temperature range will make the timer start timing and then set off some sort of alarm.

low limit < high limit
<----------FALSE|low limit--TRUE--high limit|FALSE------->

high limit < low limit
<----------TRUE|low limit--FALSE--high limit|TRUE------->

Thanks! That explains it!

Actually, the LIM instruction tests are...

TestValue >= Low Limit AND <= High Limit.

At the Low and High Limit values, the output will always be true. It's that "or equal to" in the comparisons which makes the difference.

As a result, you cannot "invert" the operation of a LIM by swapping the Low and High limits.

Examples....

LIM 3 TestValue 5 will be true for TestValue = 3, 4, and 5, false for all other TestValues.

LIM 5 TestValue 3 will be false ONLY when TestValue = 4, which is NOT the inverse of the case above.

and just to nail it all down, regardless of how the limits are set up, whenever you're RIGHT ON THE LIMIT then the LIM instruction will be evaluated as TRUE ... specifically, reversing the LOW and HIGH settings will indeed reverse the rule for "between" and "outside" test values (as already mentioned) ... BUT ... whenever you're "dead on the money" with either Limit setting, then you'll always get a TRUE condition ... specifically, the "dead on the money" rule does NOT reverse ...

and one more tip ...

in some cases, using VARIABLES for the LOW and/or HIGH limits can cause "sudden surprises" in operation ... for example:

suppose that the HIGH LIMIT value is being taken from an analog sensor in the field ... suppose that this value is NORMALLY expected to be much higher than the LOW LIMIT ... everything works perfectly for a very long time ...

now suppose that a wire in the analog input circuit gets broken ... OOPS! ... suddenly the "High Limit" setting becomes LOWER than the "Low Limit" setting – and the LIM instruction REVERSES its operation ...

all of a sudden, things which were supposed to stay OFF - come ON ... and things which were supposed to stay ON – go OFF ...

letting an operator enter the High and Low Limit values from an HMI device can also scramble the eggs ... some experienced (been burned before) programmers will use ladder logic to "clamp" the High and the Low Limit values to keep everything on an even keel ...

EDIT: looks like daba is typing faster than I am – but we're on the same track ...

Thankyou...very clear now!