Is there any reason why someone will write this logic?

Anibal Hernandez · Apr 12, 2004

Is there any reason why someone will write this logic?

---NO %Q23-------NC %Q23-----------------------------Coil %Q24

BTW: How do you use the LADDER button to post ladder logic?

Andy

rdrast · Apr 12, 2004

One reason to write something like that, or edit that into logic, is to disable a rung. Either due to a feature not included in a system, or for diagnostic purposes.

That way you can turn off a particular rung, without having to delete it. In AB higher level PLC's, they have an "AFI" or always false instruction. Other PLC's may have always false bits, or in some cases, the programmer may include always false (and / or) always true bits as well to serve in cases like this.

Anibal Hernandez · Apr 12, 2004

rdrast:

Even though the Rung will be true if output %Q23 fails in the ON state?

For what kind of diagnostic will it be used?

rdrast · Apr 12, 2004

How in the world could the rung be true?

You are saying basically:

Output = IN * NOT_IN

If IN is true, NOT_IN will be false. 1 * 0 = 0 or off.
If IN is false, NOT_IN will be true. 0 * 1 = 0 or off.

And that construction is used to disable LOGIC, not to prove or disprove I/O.

Anibal Hernandez · Apr 12, 2004

Thanks rdrast good point!!

abhilashponoli · Apr 12, 2004

how come no replys for anibal's 2nd question????

Anibal Hernandez · Apr 12, 2004

abhilashponoli:

I tried copy and paste but did not worked, I have seen ladder logic image file right inside the post but I don't know how is done.

Perhaps someone can help.

Thanks

ArikBY · Apr 12, 2004

You should click on LADDER botton then write your ledder between {LADDER}and {/LADDER}



-------(1)-- i-------(O)

             i 

---(b)-------i

Thats it

kamenges · Apr 12, 2004

When you click the LADDER button a pop-up appears. If you don't enter anything in this pop-up the LADDER HTML tags won't get inserted in your message (at least in my case). That can be a bit confusing.
When then pop-up comes up just put a letter or something in the text line and hit OK. That will insert the tags and you can edit in the main window from there.

Keith

Rick Densing · Apr 12, 2004

Anibal Hernandez said:
abhilashponoli:

I tried copy and paste but did not worked, I have seen ladder logic image file right inside the post but I don't know how is done.

Perhaps someone can help.

Thanks

The images are a screen capture of their desktop. Crop and convert to a .jpg file. Then use the attach file field at the bottom of the screen to attach the picture.

Anibal Hernandez · Apr 12, 2004

Rick Densing:

This is the way to go, I love it.

Thanks

panic mode · Apr 12, 2004

As rdrast mentioned, AFI instruction in RSLogix5000 is
comparable way to disable logic but in my opinion it is best to leave the
AFI out of program. There is no damn crossref for it...

Terry Woods · Apr 12, 2004

Type in your text in Text_1, then type in "LADDER" (upper or lower case) enclosed by square brackets as shown below, then enter your ladder. After your ladder, type in "/LADDER" (upper or lower case) enclosed by square brackets as shown below. Then enter more text in TEXT_2.

Repeat process to enter more ladders and more text.

Text_1

[LADDER]

  Q23       Q23       

--| |-------|/|-----( Q24 )  !Regardless of the state of Output 23, Turn OFF Output 24.

This is one of the ways available to FORCE an output bit OFF.



  Q12       I16       

--| |---+---|/|-----( C18 )

        |

  C18   |

--| |---+

[/LADDER]

Text_2

With respect to the Q23 code, the first question that occurs to me is... Why in the world would anyone use an Output to drive an Output?

To do so is a sign of a very poor process design(er).

rdrast · Apr 13, 2004

Terry, about the only reason I can think of for actually using an output to directly drive another output is where the PLC ladder logic doesn't allow for output branching. I know I often have multiple output's (say for run pl's on differnt operator stations).

In that case, it could save duplicating a lot of conditional input logic, and would be fine as long as they were consecutive in the program.

-- edit
Or perhaps they are different classes of outputs, 120vac, 24vdc, relay... but still, only if the CPU doesn't support output branching.

Rick Densing · Apr 13, 2004

rdrast said:
but still, only if the CPU doesn't support output branching.

I would have the logic rung drive an internal bit and the have that bit drive each output in its own rung.

Is there any reason why someone will write this logic?

Anibal Hernandez

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