Nested Subroutines at free version.

KirillIvanov · Sep 7, 2019

Hi all.
Please see attached.

Tried to google the error with absolutely no results.

I did the same at payed version and real PLC so I'm almost sure it's some free version or Emulator causing the problem. Just curious if someone knows about this limitation.

The subroutine suppose to call itself again and again with every True condition during 8 checks. 1,2,3 JSR called with no issues. When it's equal or more than 4 I have this error which apparently comes from RSLogix 500 Emulate.

VirtualBox_Windows 7_07_09_2019_18_03_17.png

KirillIvanov · Sep 7, 2019

More informative screenshot.

VirtualBox_Windows 7_07_09_2019_18_06_26.png

dmroeder · Sep 7, 2019

I've never tried this, but are you, for example, trying to jump to subroutine "A" within subroutine "A"?

KirillIvanov · Sep 7, 2019

Yes. Same subroutine.

It checks eight conditions with one shots if TRUE and fills FFL with values.

Perhaps there is a better way to make a FOR cycle at Ladder Logic?

Here is the code.

VirtualBox_Windows 7_07_09_2019_18_31_38.png

dmroeder · Sep 7, 2019

Although you can call subroutines from within other subroutines, you shouldn't be calling the same one within itself.

Operaghost · Sep 7, 2019

Two issues.

1. There is a limit to how many nested subroutines the SLC supports. It varies by controller and is either 4 or 8.

2. If you are having the subroutine call itself it will potentially fail the watchdog timer. Too much detail to go into here.

Use a JMP instruction at the location you want to jump from (current JSR) and put a LBL at the beginning where you want it to land.

Using the JSR method it is actually suspending the subroutine scan and starting a whole new scan. Then it suspends that and starts another scan.

Using the JMP/LBL method you are within that one scan only and just jumping back to the top to rescan the code there. Much less overhead and no "nested" limits.

OG

KirillIvanov · Sep 7, 2019

Operaghost said:
Two issues.

1. There is a limit to how many nested subroutines the SLC supports. It varies by controller and is either 4 or 8.

2. If you are having the subroutine call itself it will potentially fail the watchdog timer. Too much detail to go into here.

Use a JMP instruction at the location you want to jump from (current JSR) and put a LBL at the beginning where you want it to land.

Using the JSR method it is actually suspending the subroutine scan and starting a whole new scan. Then it suspends that and starts another scan.

Using the JMP/LBL method you are within that one scan only and just jumping back to the top to rescan the code there. Much less overhead and no "nested" limits.

OG

Thanks for your answer.

That exactly what I did at the beginning but it didn't work after a first jump. Probably my code was wrong. Is it possible to use a same jump address for few different rows pointing same label or every jump-label should be unique?

UPD Nevermind... Already found at JMP help that few jumps may point to the same label. Thank you.

bernie_carlton · Sep 7, 2019

Give consideration that your logic could take place over multiple scans. I have written applications with lengthy string searches. Working through an array of target strings I broke it up to searching only 10 strings each scan. Even with 150 targets the search took almost no noticable time. I used the Jmp/Lbl pairs within each scan code for the 10 searches.

Nested Subroutines at free version.

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