Subroutines

I'm currently in college, taking a PLC course using AB SLC 500 series. We have this problem where we are to take an analog signal (1-5V) scale it, from 0-100%, Subroutines are required to process under and over range values. Thought I'd use LTE and GTE commands to send it to the appropriate subroutine. Problem is in the subroutine we must have a bliking O/P and an acknowledge I/P so that the LED goes solid. When the variable goes between the set values the alarm condition should clear itself off. I used a Limit command to do this but if the O/P was put to steady state by acknowledging the alarm the O/P doesn't reset before exiting the subroutine. How do I solve this?? do I clear it in the subroutine or on the next rung in the program??

Congratulations! You've learned something. To wit: When you stop SCANNING an output (by no longer calling the subroutine), the output stays in the last state.

I suspect that's why your teacher had you use subroutines. Well done!

So, your question is, "how do you solve this?". There are several ways, some better than others.

You could, as you've suggested, move the light logic out of the (dead) subroutine. This might not work, however, if you use conditions that the subroutine controls on the rung that's outside of the subroutine.

Since you need to call the subroutine in order to control the light, one solution is to call the subroutine when you have EITHER an over-range condition, OR the light is on.

Another technique might be to create a "In range" subroutine, which would also control BOTH lights (turning them off). This "double coiling" technique is frowned upon by most programmers, but as long as you are ABSOLUTELY SURE that one and only one subroutine can be on during the same scan, then you won't have any conflicts which can lead to sudden hair loss ("All the conditions to turn the light ON on this rung are true - why isn't the light coming ON ?!?"

There is no "right way" - there are only better and worse ways. "Right" means that it works. "Good" means that someone else (or even you) can understand what's going on, and change things without unexpectedly changing something else.