multiple outputs

I had a minor argument with a fellow tech this afternoon about multiple outputs on a rung. It went something like this:

---|X1|---------------|--------------------------(Y0)

------|X2|----------------(Y1)
Y1 is "or'd" out with Y0 on the same rung. I have seen this on many rungs on this forum and see nothing wrong with it. My associate says that there should NEVER be more than 1 output per rung, ever. When I asked "Why?", he replied that it was bad practice. My question is: Who's right?
Does anyone have an opinion?

Like probably 90% of my answers....
It depends.

That being said, I often have multiple outputs per rung, provided the PLC supports such constructs. If the PLC does not, I often find I need to drive an internal bit with state logic, then use that bit to drive 'parallel' outputs.

Why? Two reasons come to mind right off the bat, one, I may need to drive several devices that have too much load for a single output, and I prefer not to use interposing relays if possible. (Light and Horn?)

Or, I may need to drive a DC and and AC output. (24V control enable and 115V solenoid?)

Or, I may want to update an HMI with the current status of an output. Directly reading the output table with an HMI is often inefficient, so I use an internal flag 'State' bit, and a physical output.

I don't see the practice as inherently 'Evil'. I use PLC's to accomplish tasks, in the most efficient manner possible. Using an internal state bit, to drive an output on a seperate rung incorporates many more instructions that are unnecessary.

Do I do it just because I can? No. I do it when the situation warrents it.

YMMV

Just a thought. I have seen lots of logic where the programmer is moving an entire word or more to the outputs on 1 rung.

That is multiple outputs on a rung.

I don't see much wrong with it, I think you meant it to be like this?




But you also said..."or'd" to me a OR would be this X1 or X2 would make Yx true... but what ever... I also will write a alarm series like rdrast said, same as above but no X1, if needed


PS, how's the SEW working these days?

Depends what you're doing, if you have multiple outputs it may seem uneconomical as you've turned on or off more than one bit from the same logic.

BUT.

In sequencers, you may initiate different things in the same step and complete (reset) in different steps.

You can have multiple starts for different sequences.

Multiple resets.

Switching flags for logic and data bits for HMI/SCADA displays.

Some people like to switch an output and a seperate flag for internal use.

In answer to the question, its not bad practice, unless your using the same type of bits for identical purposes where one flag would do.

In my opinion, If you have many outputs enabled by a (or some) conditions, then:

1- program-wise: it will be economical to combine all of them into one network to save space and to simplify the program.
2- Electrical-wise: I once had a problem that i energized many outputs at the same time which made the main circuit breaker (100A) to cut and on other project, the relay-feeding fuses were cut. So take care.
3- Programming-wise: I don't think it's a bad practice to enable many outputs in the same network.

AMAZINGAHMED said:

2- Electrical-wise: I once had a problem that i energized many outputs at the same time which made the main circuit breaker (100A) to cut

and on other project, the relay-feeding fuses were cut. So take care

.

Click to expand...

For long conveying systems etc, it the norm to have a small delay between each output to ensure you don't strain or trip the power source.

Alendi had a recent post with this rung with multiple outputs.

I like it.

Not that there is anything wrong in the above post with the nested JSR's but nesting takes a longer time to scan than if you were to use parallel branches.

LJBMatt said:

Not that there is anything wrong in the above post with the nested JSR's but nesting takes a longer time to scan than if you were to use parallel branches.

Click to expand...

I thought nested and parallel branches were the same thing. Do you mean sequencial networks (segments).

In AB logic that is above there is a difference in appending a rung branch and extending a new branch down. I believe when adding a branch (nesting) you are limited to either 5 or 7 branches, but you don't have that limit when you extend a parallel branch.

In this example after you append more than 5 branches you exceed the nesting limit of the AB processor.

In this example by extending branches making them parallel (on the same level, not nested) the rung will verify. It does take the processor more time to process a rung with nested branches as opposed to parallel branches.