Explain why this code is used.

willie · May 2, 2003

This is a code that I have seen quite often and have never used. Is it a form of latching a bit or output? I'll try to draw what I am asking. B:2/0 is used in the same rung. As and or under I:0/1 input.

I------ I:0/1 -------------------------------- B:2/0 ---I
| [] | ( )
| |
| |
-- B:2/0 --
( )

Thanks guys;
Willie

seppoalanen · May 2, 2003



 I------ I:0/1 -------------------------------- B:2/0 ---I

     |     []    |                               ( )

     |           |

     |           |

      -- B:2/0 --

          ( )

Try open this as quote (bottom-right corner), and you can see quotations for fixed text, I hope it works..Yes! Is that B:2/0 allways on ?

willie · May 2, 2003

Ladder diagram

So if this is used in a program the program will run continuous? How would you stop it, with an unlatch bit? I'll try the ladder diagram again. Hope it works.

I------ I:0/1 -------------------------------- B:2/0 ---I

Thanks;
Willie

seppoalanen · May 2, 2003

How that B-bit have used? Is it for previous scan state of that Input, f.e. ONE SHOT purpose ?

willie · May 2, 2003

I am not understanding your response seppoalanen. I am just asking why a rung program is used this way?

Willie

How that B-bit have used?

rsdoran · May 2, 2003

Seppo was asking how it is being used in the program. Its possible that its condition is to be always ON if I:0/1 activates.
BUT
I do not believe this to be an acceptable rung, its needs conditioning to allow the bit to be unlatched/reset.

Steve Bailey · May 2, 2003

It might have been used as a diagnostic tool to see if the input was ever energized. If so, the programmer should have included a comment telling what it was intended for, or else deleted the rung once he didn't need it anymore.

Other than that, as you posted it, with no way to turn the coil off, it's pretty much useless.

Jack · May 2, 2003

Re:

Typically, this is used to latch a bit. A disarm instruction should proceed it such as

--I:1/1------I:1/0----------------------------O:2/0---
[ ] | [ ] | ()
| |
----O:2/0---

When O:2/0 first picks up, it is held in by itself no matter what
I:1/0 does. It is deenergized when I:1/1 is false. This assumes I:1/1 is true when I:1/0 goes true

Steve Bailey · May 2, 2003

Jack,

Your code is a widely used method of latching, but it's not what Willie posted. Of course, it might be what Willie meant to post. Here's an edited version of Jack's code. Willie, could it be what you're frequently seeing?





 Start     Stop           Run

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

        |

  Run   |

--] [---+

This ladder works when Start is wired as a NO pushbutton and Stop is wired as a NC pushbutton.

seppoalanen · May 2, 2003

Mayby my english is not clear, but I like ask 'where and how' B:2/0 have used as x-reference ? This is one explanation for why that rung could have done.



      I:0/1        B:2/0        ???

I------] [----------]/[---------(L)------I  (as ONS evalution)

.

.

.     I:0/1                    B:2/0

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

Other explanation could be that B:2-series bits have in some group for data exhange or for alarm area-bit or what ever ??

KP_EENG · May 2, 2003

The rung of code that you listed in your initial message could be used as a trap to troubleshoot a signal that turns on and off so quickly that it is difficult to monitor it with the PLC programming software because of the communications delay. This is a common troubleshooting technique for these type signals. If the programmer did not include an unlatch contact in the rung, they were probably toggling the bit off manually once it was set.

garryt1 · May 3, 2003

Hi Willie,

This bit looks more like a pulse test or something along them lines, you have to ask yourself - is this used anywhere else in the program ? and what for? i think you may find it is being counted somewhere else in the program.
If i were to use it as a test bit i would use the following code and also as someone else suggested previously i would comment it.

B3/0
--------------------------------(L)
Always True

which for testing could be used in the either or state:

B3/0
-----|\|---------------------- rung never on.

B3/0
-----| |---------------------- rung always on.

Explain why this code is used.

willie

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