JSR(Jump to subroutine) problem

colorofwind · Jul 5, 2012

Hello, I have a simple JSR problem which confuses me. The software is RSLogix 5000.
In the main routine, I have a rung like this
A
------| |----------------|JSR|---------

When switch A is turned on, it jump to the subroutine.

In the subroutine, I turn on some coil B (not latch coil).

--------------------------(B)-----------

However, when switch A is turned off, coil B in the subroutine is still energized. Why can this happen?

Please share your thought, thanks a lot!

Oakley · Jul 5, 2012

Because there is no condition that makes it not true.

It stays in the last state because there is nothing to turn it off.
Turning off switch A is nothing more than telling the controller not to scan the routine - does not reset anything.

dhuber · Jul 5, 2012

When switch A is turned on it scans the subroutine and writes a 1 to "B".
When switch A is turned off, it does not scan the subroutine, it doesnt write any value to "B", even though "B" in your mind is false, it is still set in memory, and there is nothing writing to "B" to tell it to turn off.

Hope that helps.

Mickey · Jul 5, 2012

That is one of the main gotchas when using conditional subroutines. Try to avoid them if you can.

colorofwind · Jul 5, 2012

dhuber said:
When switch A is turned on it scans the subroutine and writes a 1 to "B".
When switch A is turned off, it does not scan the subroutine, it doesnt write any value to "B", even though "B" in your mind is false, it is still set in memory, and there is nothing writing to "B" to tell it to turn off.

Hope that helps.

Thanks guys! It makes sense. But how can I turn off coil B in this situation?

Mickey · Jul 5, 2012

Whatever rung coil "B" is on needs to be scanned false. Remove the condition for that subroutine. Is there some special reason you need a conditional subroutine?

colorofwind · Jul 5, 2012

Mickey said:
Whatever rung coil "B" is on needs to be scanned false. Remove the condition for that subroutine. Is there some special reason you need a conditional subroutine?

We compare reading from thermocouple to some constant temperature settings and then choose which mode PLC should work at. Each mode is a subroutine. Then we use the output as an input to trigger the subroutine, like this:

------|CMP|------------------(Mode A)-----

------|Mode A|----------------|JSR|--------

I'm new to PLC and there must be some better ways to do it.

Mickey · Jul 5, 2012

If you want mode A's subroutine to rung and not mode B,C..., then I would put something
on the rungs within the routines you don't want to run to make them false.

colorofwind · Jul 5, 2012

Mickey said:
If you want mode A's subroutine to rung and not mode B,C..., then I would put something
on the rungs within the routines you don't want to run to make them false.

Yes, you're right. I put a switch before B in the subroutine instead of before JSR. Now it works perfectly. Thank you so much!

Mickey · Jul 5, 2012

You're welcome.

JSR(Jump to subroutine) problem

colorofwind

Lifetime Supporting Member

Oakley

Member

dhuber

Member

Mickey

Lifetime Supporting Member

colorofwind

Lifetime Supporting Member

Mickey

Lifetime Supporting Member

colorofwind

Lifetime Supporting Member

Mickey

Lifetime Supporting Member

colorofwind

Lifetime Supporting Member

Mickey

Lifetime Supporting Member

Similar Topics