example 4-20mA represent 900-1000unit (after doing scaling). i have a set point at 950. if reading below 950(using Compare instruction), i
give alarm output bit=1. However, if it fall below 3.5mA or above 21mA, i am suprise i get reading above 1000. hence alarm bit=0.
i have put in extra compare instruction to compare if reading above 1000, move #0 into the reading register. hence when this condition is
TRUE, #0 is move into reading. But once #0 move into reading, the condition become FALSE. and hence the condition will toggle.
am i doing it wrongly?? any advice??

 

Instead of moving a 0 in if it goes over range, move a 1000 in, and set a flag elsewhere in your program to know that it is over-range.
That way you won't get your under-range error.

 

If you can, why don't you change your scale a little bit so that you will account for signals bellow 4mA or above 20mA. Doing so, you know
that your Scale value will always be withing the range and it won't offset to a crazy number because it is falling off your scale.

Good luck

 

Which PLC are you talking about?
Some PLC's have analogue input modules that can be programmed to give a high level alarm in case of open or short circuit.