RSLogix 500 scan times

[cekann 0]

Greetings, Is it better form to have several related instructions on a nested rung or spread out over several rungs? Does one or the other effect the scantime of the program? Thank you for your attention, Chris Kann York, PA

[Spedley 0]

There are no overheads involved in multiple rungs but there are with multiple/nested branches, however if conditional logic has to be repeated for each rung this may have an impact on scan times and memory usage. I usually group similar rungs in branches to make a program easier to read. A program is often written more efficiently when it's easier to read so this would probably cancel out any negative effect from the overheads. Edited 12 Jun 2006 by Spedley

[BobLfoot 301]

One often overlooked style factor to use when concerned with scan time is the structure of the rung. Place your XIC and XIC instructions to the Leftmost positions of the rung. They take the least execution time and can get you too the next rung sooner. Place your higher level isntructions like EQU, GEQ, LES and such in the middle of the rung. These take longer to execute than a simple XIC. Structure of this method will give you a program which "flys" thru inactive or false rungs. Of course the swing in scan time from all rungs false to all rungs true will be the greatest than with other formats.

[Stupidav 1]

I can't remember the proper terminolgy, I just know what the system sees. Both Nested and Seperate Rungs take up more memory than an Extended Branch. As an Example: Nested with 5 Layers BST XIC I:0.0/0 NXB BST XIC I:0.0/0 NXB BST XIC I:0.0/0 NXB BST XIC I:0.0/0 NXB XIC I:0.0/0 BND BND BND BND OTE O:0/0 Extended with 5 Layers BST XIC I:0.0/0 NXB XIC I:0.0/0 NXB XIC I:0.0/0 NXB XIC I:0.0/0 NXB XIC I:0.0/0 BND OTE O:0/0 5 Seperate Rungs (I can't remember what the data is between the rungs) XIC I:0/0 OTE O:0/0 XIC I:0/0 OTE O:0/0 XIC I:0/0 OTE O:0/0 XIC I:0/0 OTE O:0/0 XIC I:0/0 OTE O:0/0 And I agree with BobLfoot, the Structure of each rung can vary the scantime as well. This holds true for almost every PLC, not just the SLC 500 series, but all of ABs and others.

[cekann 0]

Thanks to all for their replies. Each one to date was helpful. Chris Kann