Reuse of ONS tags in 5000?

kolyur

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I'm troubleshooting a program in RSLogix 5000. I've located the trouble area but have come across something I don't quite understand. The programmer reused the tag from a ONS in a contact on the next rung. My background is mainly 500 but I don't see the purpose of this. Does 5000 function differently than 500 as far as one shots are concerned? On further inspection I found that this logic exists multiple places in the program so I don't believe it's a mistake. The problem I'm troubleshooting is intermittent so I don't think this is the cause of it, just curious about what's going on here.

ons2.jpg
 
On the XIC, it's looking at the storage bit of the ONS instruction. If you go into [Help-Instruction Help] there's a flow chart of how ONS storage bit works. Basically it will be set to 1 when the condition preceding it turns true and stay at 1 until the preceding condition turns false again. In other word, the storage bit acts like an OTE -( )- instruction.

I personally won't do logic like that for pure readability reason.
 
EDIT: Oops, sounds like I may have been incorrect.

EDIT EDIT: I was correct after all. The XIC will represent the output of the ONS. So it will only be on for one scan when the ONS transitions to true. The middle rung will never completely go true since it can only be true for one scan. The Timer enable bit will probably go true for one scan, but will never really time.
 
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Not the worst logic I seen. Right now I'm going through programming done by one of the major US engineering firm... latches everywhere and I also found multiple double OTE plus AOI that ask for latch to turn on, turn off something... I have to go to the bathroom and hit my head against the wall every so often.
 
I take it that the XIC will be on as long as the rung is true, the status of the ONS would stay high until reset, unless CL runs differently than PLC5 & SLC.

Thus the timer would flash for as long as the top rung was true.
 
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Aabeck said:
I take it that the XIC will be on as long as the rung is true, the status of the ONS would stay high until reset, unless CL runs differently than PLC5 & SLC.

Thus the timer would flash for as long as the top rung was true.
Click to expand...

The ONS will generate a pulse for one scan when it transitions to true. The XIC will represent that pulse. The XIC will only be on for one scan.
 
An interesting thread. I believe that the bit referenced in the ONS instruction will stay TRUE as long as the logic leading to it is TRUE. Only the logic state passed to the right out of the ONS instruction is turned ON for one scan.

The reference to it using the XIC instruction would remain TRUE during that time.

But how about someone enter this logic into an RSLogix 5000 compatible system and report back on the actual operation?
 
I agree with Bernie's interpretation, meaning the storage bit will reflect the state of the logic preceding the ONS instruction.
Since we're talking about relay logic, let's think about the instruction in terms of relays. You can think of the ONS instruction as creating a standard relay with one special contact (One-Shot) that is true for one scan when the coil first energizes. All other contacts from that relay behave as they would in a standard relay. The special (One-Shot) contact is used for the output of the ONS instruction.

Someone with an available Control Logix or Compact Logix PLC could try entering this code


 Logic       ONS_TAG           ONS_RUNG_OTE

--] [--------| ONS |------------------------( )-



ONS_TAG                          XIC_RUNG_OTE

---] [---------------------------------------( )-


If Bernie's interpretation is correct, when monitoring the logic online if "Logic" is true, the ONS instruction should be highlighted, but "ONS_RUNG_OTE" won't be while "XIC_RUNG_OTE" will be.
 
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I think I agree with Bernie. And that is how PLC5 & SLC's work.

The OTE on the top rung will only be true for one scan, but the XIC in question is looking at the retention bit the ONS instruction uses, and IN RS5 & RS500 the XIC would show the retention bit ON as long as the rung was true.

I will have to go online with a CL Monday to confirm or disprove this, as I don't have an extra one at home for $ome rea$on.
 
bernie_carlton said:
But how about someone enter this logic into an RSLogix 5000 compatible system and report back on the actual operation?
Click to expand...

EDIT: I'm totally an idiot on this one, please ignore me. I have a Compact Logix in front of me and the others are correct, it stays on as long as the ONS is true. I made a stupid typo when I tested it.

Again, my apologies, sorry for the confusion.
 
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A picture is worth a thousand words.
Using Steve Bailey's suggestion:

CLogix_ONS_XIC.jpg

This is RS5000 ver.20 in a ControlLogix L75.
 

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