Talk:Ladder logic

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The analogy between logical propositions and relay contact status is due to Shannon I would say Mr. Bool (Boolean algebra) plus Mr. de Morgan (de Morgan's laws) would be here more appropriate.


...I put in a few extra details and had a bit of a hack around- let me know if you want me to expand it some more - Scampiandchips

Contents

[edit] if you want...

...I could take some screen shots of your logic as implemented in Rockwell programming software and post 'em here...

[edit] nand?

is this

  ---[\]---[\]--- 
      X     Y

really X NAND Y ?


isn't it (NOT X) AND (NOT Y) which actually means X NOR Y?

Yes, you're right. Fixed.

[edit] standard contact symbols?

The article currently has a mix of contact symbols:

 --[ ]--
    X

 --| |--
    Y

 --][--
   Z

They all mean the same thing, right?

Should we standardize on one type ? The --| |-- seems least likely to interfere with Wikipedia formatting (so we don't need <nowiki></nowiki> tags).

(Note that the --( )-- "coil symbol" at the right of the rung is *supposed* to look different -- it does mean something different).

--DavidCary 18:06, 25 Jun 2005 (UTC)

Agreed - I'll admit I was a bit confused by all the different symbols to begin with.--Rehnn83 14:10, 22 September 2006 (UTC)

[edit] Normally Open COIL?

What the heck does this mean? A coil is just an electomagnet. It can have contacts that are NO or NC but it can't be either. right? TomCerul 18:11, 3 August 2005 (UTC)

This is probably better called a "not" coil - if a not coil is energized by a rung, its normally-open contacts are open and its normally-closed contacts are closed (a regular coil will have NO closing and NC opening). I ran across this on a Reliance Automate 35, where it was extensively used to save memory - though it made the resulting programs very hard to read (especially so as there was no documentation or variable names displayed by the programming panel that could have alerted you to the weirdness). I had to write my own documentation software for that machine as we only had the one and the manufacturer didn't support anything better. I'll see if I can fix the text in the article. --Wtshymanski 17:41, 4 August 2005 (UTC)
I agree, it's a NOT coil. The proper use of NOT coils arises from the concept of "failsafe". One example is in machine tools. There is a circuit called an E-STOP string. All of the safety elements on the machine will have a set of NC contacts and be wired in series. The hydraulic pressure is NOT too low. The safety gates are NOT open. The drawbar that holds the cutter is NOT loose. If NO contacts were used, the control wouldn't be able to tell if a wire in the string were broken. In the old days, the string would control a relay. The output of that relay would go immediatly to another relay to be negated. Seán 03:51, 13 March 2006 (UTC)
Odd. We did failsafe design but we'd just use the other set of contacts on the relay. We ended up saying "Normally Open, Held Closed" a lot. TomCerul 21:36, 20 September 2007 (UTC)

I agree with TomCerul - a "normally open coil" is a misnomer. Coils are neither open or closed. It's the contacts that are normally open or normally closed.

[edit] Latch Example

Someone added something to the last example on latches, and it sounds contradictory. It should be fixed for consistency. I'm pretty sure the first start/stop button figure is accurate. - ninjayeti

[edit] Graphics

Just made a few minor changes and additions. I can make graphical examples of the ladder logic shown in the article. However, this would increase the load on the servers, and the present ASCI graphics seem to be sufficent. Also, I am considering re-writing the article to be more streamined and organized. Thoughts?Dudecon 18:26, 4 April 2007 (UTC)

Decent SVG graphics (or at least PNG) would be appreciated. I wouldn't worry about the server load. If you feel like fixing the article, do it. You might want to see the PNGs on the Spanish and Italian versions of the ladder logic page. —Pengo 09:31, 24 July 2007 (UTC)

[edit] Orientation of ladder diagrams

The name is based on the observation that programs in this language resemble ladders, with two vertical "rails" and a series of horizontal "rungs" between them. Ladder logic is popular in North America while in Europe it is more common to draw horizontal rails along the top and bottom of the page and vertical rungs sequentially from left to right.

Really? PLC programming software is not different (text localization aside) for American and European markets, and all the systems I've used (in Europe) draw and print out programs in the same way, with vertical rails. Horizontal rails are perhaps more common for electrical schematics in Europe, but no-one cares that ladder is drawn in the other orientation. --DavidHopwood 23:44, 28 August 2007 (UTC)

[edit] merge "relay logic" into "ladder logic"

What do you think about merging "relay logic" into "ladder logic"?

The "relay logic diagram" on the relay logic page looks almost exactly like one of the "ladder logic diagrams" on the ladder logic page.

As far as I can tell, the subtle difference between the 2 heavily overlapping terms is:

  • "relay logic" implies that something really is built with physical relays (almost always designed using "ladder logic" diagrams).
  • "ladder logic" is a style of schematic diagram, standardizing the layout of "relay switches" and "relay coils". Any particular ladder logic diagram could be implemented with physical relays, but nowadays is more often emulated with a microprocessor interpreting the diagram and simulating the relays.

In this case, the thing, the diagram describing the thing, and something else that simulates that thing, seem closely related enough to cover all in one article.

--68.0.124.33 (talk) 16:47, 19 February 2008 (UTC)

I do not think it is a good idea to merge relay logic with ladder logic, since they are two distinct applications of control theory. Ladder logic is used primarily for PLC applications and represents one of four main programming methods for PLC. Ladder logic is restricted to very basic symbology for device representation (i.e. coils and contacts for outputs and inputs). Relay logic is used to graphically represent a control circuit schematic diagram and utilizes a much wider array of symbolic references (i.e. motors, solenoid valves, etc.) Carl142 (talk) 17:12, 25 February 2008 (UTC)

Retrieved from "http://en.wikipedia.org/wiki/Talk:Relay_logic"

If one is a restricted form of the other, then I think we should merge them -- in the same way that "hicube boxcar" is one kind of boxcar, and both are described in the same encyclopedia article. --68.0.124.33 (talk) 23:00, 24 March 2008 (UTC)

Relay logic is not a restricted form of ladder logic. They are completely different entities. One is a hardwired, traditional method of control interface, while the other is a software programming language requiring a microprocessor-based control system. There have been countless books written on this subject by authors such as Simpson, Stenerson, Petruzella, and every one of them concurs that relay logic and ladder logic are completely separate examples of control system technology. Merging them would not only create confusion, but it would be highly inaccurate.38chad (talk) 13:04, 26 March 2008 (UTC)

No merge. A computer language and logic family are different things. Ray Van De Walker (talk) 19:30, 3 April 2008 (UTC)

No merge. Agreed. Ladder logic != relay logic. I can write a program for an L63 processor, but I can't wire an enclosure. That's the difference in my eyes. You folk who actually know what you're doing can handle the relays and switches, us juniors can do the programming. Tarkaan (talk) 10:17, 11 May 2008 (UTC)

Absolutely agree with all of you guys above - don't merge. Just because they LOOK the same, doesn't follow that they MEAN the same!!! However, what _can_ be appropiate would be to add a note somewhere in relay logic saying that the syntax of the ladder language has been highly inspired by relay logic, or something like that, as that is really the case. Or just mention ladder under "See also" in that article. 130.237.57.80 (talk) 14:55, 16 May 2008 (UTC)

[edit] The "not" symbol

So far, in the examples on this page, you've been using --[\]-- as a NOT, when all the ladder packaged I have used (DirectSoft, AllenBradley..) use a forward slash, as in --[/]--. Even the example picture uses forward slashes for NOT. Should this be fixed? --Mainstreetmark (talk) 18:48, 20 May 2008 (UTC)