LOGIX 500

[Jarrodharms75]

I have been doing logic programming and troubleshooting for about 25 years.
I started working for a new company as PLC Application Engineer and I am trying to work through this program and I am seeing the use of the same output latching throughout program latching in 4 different places and I have always been under the impression to not use duplicate outputs in the code. Am I missing something?

 

[MikeyN]

Welcome to the forum! You will definitely get better answers than mine if you post this in the main forum. Perhaps describe the issue that you are trying to solve in that post as well.

Generally, you don't want to use duplicate outputs, but without seeing the program it is hard to say if this is an issue or not. The use of latches and unlatches don't seem like they would lend themselves to the same problems that simple OTEs would. Generally, if an OTL is latched, it stays on until an OTU is energized. If an OTE is energized early in the scan, but then a second OTE is not, then the output won't turn on at the end of the scan.

 

[OkiePC]

MikeyN covered most of it, but the one more reason to use latching instructions is to keep track of a condition that you want to survive a power cycle of the CPU.

 

[Steve Bailey]

If the subroutine jump instructions to files 3, 4, and 6 are such that only one of those is called at any given time, it's not necessarily a bad thing. It all depends on how the logic handles the change from calling one file to calling either of the others.

 

[Operaghost]

Agree completely with Steve's comment. Reusing an output with latches (OTL/OTU) is less of an issue that when you use OTE type instructions. However, you still have the last rung rule to deal with. And it is probably poor organization to have rungs 12 and 14 (LAD 3) both use the same OTU output. They should have found a way to get that all into a single rung with one OTU.

OG

 

[robertmee]

I've used OTU/OTL combos like that in SLC500s for implementing grafcet/,sequential sequencers. The general form is:

Xic b3/0 (step 0) xic some condition, OTL b3/1 (step 1), OTU b3/0 (step 0).

This steps the sequencers from step 0 to step 1. The benefit, is I can have 16 steps in a single b3 word and at a glance know exactly what step a process is in.