Branch nesting question

tacm

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Join Date
Jan 2008
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usa
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105
Hi guys, What is the maximum number of output branch nests in rslogix 500 using a micrologix 1500 processor?
Thanx, Steve
 
That depends on how you are branching. You can nest four branches in the ML1500. Extended branches are better than nested branches - use nested branches only when you need logic within the branch. There isn't a limit on extended branches.

A110608A.JPG
 
You are allowed four levels of nest. So you get your output and then four levels.

However, as mentioned.....why nest? Any nest can be re-written as a parallel (extended) branch.

OG
 
Incidentally, if you are using a ControlLogix then don't branch at all. Place the outputs in series, it will execute faster because there is no branching and fewer instructions to run.

This rung: ----( )----( )----( )----( )-
has five fewer instructions that one with branches.
 
No-one has yet pointed out that NESTED branches use more instructions than EXTENDED branches. Branch Starts (BST) and Branch Ends (BND) are instructions stored in processor memory.

More instructions means more memory used, and worse still, a longer execution time (more instructions to process).

In Alarics post #4, the code stored for the first rung (using extended branches) is....

BST XIC B3:0/0 NXB I:0.0/0 BND BST OTE B3:0/1 NXB OTE O:0.0/0 NXB OTE O:0.0/1 NXB OTE O:0.0/2

And for the second rung (using nested branches) is...

BST XIC B3:0/0 NXB I:0.0/0 BND BST OTE B3:0/1 NXB BST OTE O:0.0/0 NXB BST OTE O:0.0/1 NXB BST OTE O:0.0/2 BND BND BND

The "extra" instructions are in red, these waste memory and processor execution time. There are 12 instructions in the first rung, and 18 in the second, that's 50% wastage !!

Also, on the larger machines (that allow more than four levels of nesting), the rung eventually goes wider than the screen view, making the logic hard to follow. Using Extended branches adds nothing to the rung "width".
 
Last edited:
Just tried it on a SLC 5/04, looks it allows the main output plus 4 nested branches, does that count as 4 or 5?

Wow, really? In a 5/04?

I never nest branches when the logic is equivalent, I think you can have 16 extended branches in a SLC 5/0x, not sure about the Micrologix models. I suppose my home copy of RSLogix500MicroStarterLiteFreeware could tell me.

Paul

Wow, its seems each day, I know a little bit less than yesterday.
 
I remember there used to be "an undocumented enhancement" in the SLC's. If you attempted to do an on-line edit to a rung with 7 or more NESTED branche's the CPU would crash and turn into a teapot. I spoke to (back then) Allen Bradley support and there solution was don't use nested branches.
 
i have always tried to limit the extended branches to 10.
i had a 5/03 project that went nuts when i programmed in 12 branches on two rungs. i eventually created three rungs of logic and everything was fine. call me crazy, but that's what happened.

regards,
james
 

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