Limit Instruction

If I am using a LIM instruction and the low limit is 10 and the high limit is 20 and the actual tested value is either 10 or 20 does the limit become true or is it the values BETWEEN 10 and 20?

If its RS500 use the compare instruction.

Its Logix Designer actually. We are looking at values and wanting to determine what something is based off the scp instruction we have. So the LIM we use has the lower and upper value and I am trying to find out if those values are included within the window of the Limit instruction.

the simple answer to your question:

if your test value is ON THE FENCE (equal to the HIGH value or equal to the LOW value) then the LIM will be TRUE ...

as expected, any value BETWEEN the HIGH and the LOW LIMITS will also be TRUE ...

going further:

suppose that your HIGH LIMIT is 10 ... and the LOW LIMIT is 20 ... (this is what I personally call a "left-handed" LIMIT) ...

in that case the rule for values BETWEEN the HIGH and the LOW LIMITS is "reversed" ... specifically, 15 (between) would be FALSE - and 25 (outside) would be TRUE ...

so - that raises the question: for a "left handed" LIMIT does the "ON THE FENCE" rule get "reversed" too? ... nope ... once again, if your test value is ON THE FENCE (either 10 or 20 in this example) then the LIM instruction will still be evaluated as TRUE ...

going even further:

in some cases a programmer will use VARIABLES for the HIGH and the LOW entries ... that's OK - but you have to be extremely careful ... suppose that the value in the HIGH entry suddenly becomes "shifted" and becomes LOWER than the LOW LIMIT value ... (or vice versa) ... oops! ... suddenly the LIMIT instruction has become "left handed" - and all of the conditions that you intended to be FALSE are now evaluated as TRUE ...

in SOME installations this could be an extremely hazardous situation ...

Thank you.

If the low limit is = or > than 10 and the high limit = or < 20 then it's true.

This instruction can be used the other way around having the low limit higher than the high limit.

i.e. low limit 20 and high limit 10, this would then evaluate as low limit >20 and high limit <10 the instruction would be true.

I hope this makes sense, a full explanation can be found in instruction reference manuals.

Steve