one input - two outputs ...

[Ron Beaufort]

in very basic terms, for a PLC-5, SLC-500, or MicroLogix system ...



if everything else is equal, then each of the approaches shown below should give the same results ... specifically, whenever the input is ON, then both of the outputs will be ON ... whenever the input is OFF, then both of the outputs will be OFF ...


http://www.plctalk.net/qanda/uploads/cuta.JPG

[randylud]

Gee Ron, I think you are on to something here!

[Ron Beaufort]

Greetings Randy ...

this is just something from the "picture worth quite a few words" department for one of my PM buddies ... so far I haven't figured out how to attach a picture to a PM - so I stuck the information here in the practice section ...

[randylud]

Greetings to you too, my friend! I could not help taking a small jab at you for that info. Just the imp in me I suppose! Hope all is well with you and yours. I am fine here and about half way through my summer term. Plan a visit up here the next time you are in the area, I would love to show you my "new" labs. You remember what the old looked like, this is so much better.

[davev]

Yep, That is what someone with a PLC or electrical background would do. Althoug the idea of the two coils in parrellel in the same rung, I like that as as far as being straight foward and easy to understand and remember, but some customers have specifications/procedures that prohibit that. I think just because some previus PLCs could not do that, and maybe becuase of the ramifications of the rung comments or cross reference. Another reason not to do that is that if sometime later you have reason to seperate the two, then you end up adding a rung and that chages all the other rung numbers, if that is a concideration.

SOmeone with a VB background or maybe assembly/machine code background might want to have transitional (one shots)input conditions and then latches and unlatches so that it would take 6 or 8 rungs of logic and be real annoying to troubleshoot.

Oh, and then there would be someone who would want to have a function block.....

Oh, Hey, wait, this is a "PLC" forumn.

DAVE V

PS Okay, now you are going to say you were merly jesting. Or just practicing posting images.

[Ron Beaufort]

Greetings davev ...

PS Okay, now you are going to say you were merly jesting. Or just practicing posting images.

no, actually I was serious ... sometimes I have people ask me for help by using the forum's PM feature ... often it's because they assume (usually incorrectly) that their question is just too "basic" to post on the open forum ... sometimes I answer these questions - and other times I write back and suggest that they use the Q&A section because the question isn't as "basic" as it seemed - and many other people could benefit from the discussion ...

anyway ... I haven't yet figured out how to attach a picture to a PM - so I just put the picture here in the "practice area" and sent a PM to tell the puzzled party where to look for the picture ...

if my PM friend checks back on this thread, I'm sure that he'll appreciate the extra detail that you've added to the discussion ... so far he hasn't asked me WHY we might want to use one workable method over another ... all he originally wanted to know was more along the lines of "what would happen IF one output was used to control another" ...

thanks for your input ...

[Jimmie_Ohio]

thanks for your input ...And thank YOU for your two outputs!

:doh:

[Ron Beaufort]

ok ... it’s a slow day ... let’s play around just a little bit ... (Jimmy will probably assume there’s a pun there) ...



anyway ... suppose that we swap the rung order in the top example of my original post ...



http://www.plctalk.net/qanda/uploads/swaporder.jpg




now what happens when the input is first turned ON? ... will both outputs turn ON? - or not? ...



actually it depends on how fast you look at the results - and on how long you wait for the dust to settle - and how technical you want your answer to be ...



specifically, at the end of the scan when the input is first turned ON, the field device attached to output O:2/1 will be turned ON ... but the field device attached to output O:2/2 will still remain OFF ... now (assuming that the input still remains ON) at the end of the NEXT scan, then the field device attached to output O:2/2 will be turned ON ...



now to a human observer watching the field devices, it will undoubtedly appear that “both outputs came ON at the same time” ... well, actually one of the outputs was delayed in turning ON by just one processor scan cycle - which of course is usually measured in milliseconds ...



so what (if anything) have we gained here? ...



probably not much - as long as we’re dealing with real-world outputs such as pumps and valves ... but this same approach is sometimes used in ladder logic programming to give something like a “ripple” effect ... it can be used to make “scan time intensive” instructions (such as PID or other heavy-duty math functions) execute in a one-scan-after-another fashion instead of executing (hog fashion) all on the same scan ... beside “ripple” I’ve also heard it called a “cascade” effect ...



oops! ... got to go ...

[davev]

Ron,

Does the logic in your signature mean that you are always questioning?

Dave V

[Ron Beaufort]

questioning whether "to be" or not? ... no, I don’t question that anymore ... almost everything else? ... yes, always questioning ... always ...

so now we've taken a very simple picture - of a very simple PLC concept - and now we're becoming philosophical ...