what is the meaningof the digits in the followingladder logic address

dennyis

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Join Date
Apr 2003
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1
I am trying to figure out what the meaning is for the digits in the following ladder logic address. I:2/12. I know I is input but what does the 2/12 mean?
 
To break it down a little more clearly... The 2 in 2/12 tells you that the input card lives in the second card slot. (actually the third slot but the processor always lives in the first slot, called slot 0) The 12 in 2/12 tells you that it's the 12th bit.

Hope that helps.
 
dennyis,

I can't help but point out that you got away with a poorly-phrased question. You neglected to mention the brand and model of PLC you're dealing with. You got quick answers because it was obvious from your question that you are dealing with an Allen Bradley PLC, and since AB has the lion's share of the PLC market here in North Amaerica, there are plenty of people who recognized the addressing scheme.

In this case, you got lucky. In the future, when you need to ask a question, it's good practice to provide as much information as possible to help the people providing the answers. The better the question, the better the answers.
 
I've got a question for this topic.
What is the purpose to having to address an input by telling the processor everytime where the input or output is located? I've never programmed an AB plc, but have messed around with the software a little. I just never understood why it requires you to tell it what slot or location its in. Is there some added capability by addressing this way? Does this require AB programmers to constently look to see what slot the I/O is plugged into when addressing an input or output? Are there not plc paramaters to set up and tell what kind of I/O is inserted into a particular slot?
BTW,I'm not knocking AB plc's.
Tim
 
I think ya looking at this thru a mirror...ie backards.
Most plcs you have to configure the hardware in some fashion BUT in the case of AB the addressing is a product of the card's location. To me this simplifies troubleshooting in many ways...as was shown that output would be slot 2 bit 12. Many plc's more or less allow you to tell what address the cards have...I do not think this is conducive to allow simple troubleshooting. Familiarity makes things look simple to those that are familiar with them so the methods employed by other plc's may seem simple to those familiar but not those that arent. The AB format in this case makes it easy to understand and locate the card.

I reckon theres alot ta say about AB overall but this is ONE ASPECT that I think they deserve praise for..it makes one part of it simple.

I reckon I have had my share of looking for Output 16 that is on the 4th card in the rack where output 32 was on 3 card...tell me this makes sense?
 
Last edited:
rs,
Not sure what you mean by backwards and the mirror thingy. Like I said in my post, I have never programmed a AB, so sure I'm not familiar with it. I was curious to see if this method was better than another and why? Some plc's configure your card automatically, which is nice. Some people do as what you said, screwing up the order of the addresses. I to have seen scrambled addresses, but the cases I've seen was the fault of the initial installer of the units themselves not the PLC manufacturer. Some plc's have a station number setting on each I/O unit to allow you to choose the starting head address. I was just curious in why AB choose to do it this way. I am more familiar with consecutive addressing like 0,1,2,....6,7,10...etc. while not having to type in where its located. I can understand that if your trying to find a particular input and output on a rack that this would be helpfull aspect. I've not had too much trouble finding where an input or output is located on a rack unless someone didn't label the wires, but like you said this could be helpful. Just don't know, never used one. I hope one day I have a chance to get my hands on a AB plc, so I can become familiar with it.
BTW, when you do make up your wire labels and wire up something to an input or output on an AB. Do you label the wire I:2/12 or just I:12?
I can see why you probably would use I:2/12.
Tim beerchug
 
As you know I dont get to do this as much as most PLUS I have to deal with many brands. I more or learned the most of what I know from dealing with AB and the PLC5.

The plc5 offered options on slot addressing...half slot, 1 slot or 2 slot addressing. Each of these basically state what the bit addresses will be dependent on the slot it is in. An explanation of this can be found in this manual: http://www.ab.com/manuals/cp/1785621.pdf

I ran into situations many times where other plc's would allow the address to be whatever the programmer desired, I didnt realize GE Fanuc 90-30 did this until Steve Bailey pointed it out to me recently.

Its not that you have to setup an AB unit to obtain this but like most rack style plc's you setup the hardware (many newer models will auto configure) and the addressing is automatically applied to the card type in the slot, depending on type...a 2 slot addressing model if I am not mistaken (been awhile) can have an input card in slot 1 and output in slot 2 but have same kind of address..ie I:1/1 and O:1/1. I am sure Roger or some of those that deal with this on a more regular basis will clear this up.

Overall tho if you see an input card in slot 0 then you know the addresses in that card will be I:0/x etc or written similar, the way its written can be different dependent on model.

All this makes more sense to me than someone that develops a system in a rack model with a space between modules then programs...later comes back and adds a card (output in this case) to a slot before the others but the addresses are higher in sequence.

Its also possible in RSLogix for the addresses to be automatically inserted in sequence if you want that, I dont use that feature because I have a tendency to jump around when programming and "attempt" to keep addresses associated with rungs/functions.

Alas my last attempt with GE Fanuc is an eyesore, its a small program and I understand it plus have it documented where I think it would be easy to troubleshoot but its still a mess.
 
Excellent,
Thanks RS
Sometimes you just get hooked on seeing one particular approach that when you see another, your doubting its usefullness before you have a chance to understand it.
Tim
 
Tim-
I used to work for an OEM that manufactured relatively standard equipment that had several options on any given machine. Generally, any given option had enough I/O associated with it that you could remove I/O modules if an option wasn't required.
With the AB addressing method where the slot number fixes the I/O address modules could be added and removed without causing the program any real issues. On some of the auto-addressing plcs I've used in the past the I/O addresses are assigned sequentially at every processor start. In this case removing I/O modules won't work. Now, granted, in plcs where you define the addresses you can manually define the I/O addresses for each module. But in that case you end up with a system where basically each slot has a fixed starting address anyway. And if in the future you find that you want to change an 8-point input module to a 16 -point input module you may have a hassle on you hands unless you used some serious foresight in your original design.
I've used plcs with both types of I/O addressing and I like the AB method myself, mostly for the reason that Ron states. I like being able to look at a modules slot location on a control drawing and know immediately what the I/O address will be in the program without adding the like point numbers preceeding the module I am looking at to get the address.

Keith
 
Like many here, I also think AB's addressing scheme makes the most sense. But remember, with most software packages, you can "nickname" all addresses however you like... If you don't like "I:0.0/0", "I:0.0/1", and "I:0.0/2".... you can call them "Bill", "Jim", and "George".... :p

I recently did just that (though not using peoples names). I had a 90-30 on an existing piece of equipment that needed MAJOR reprogramming. I hate LogicMaster with a passion, so I decided to do my programming in DirectSOFT. All I did was (in DirectSOFT) nickname the elements with their GE counterparts. X0, X1... became %I0001, %I0002... C0, C1... became %M0001, %M0002... and so forth. After that, it was a breeze to translate the existing program to something I was more comfortable with.

This also helped a lot with the conversion of my newly written program back to GE-lish... :cool:

beerchug

-Eric
 
I pretty much only work with AB PLC's and use quite a bit of analog I/O as well as remote I/O.
I prefer to use descrete data transfer as much as possible. When using this type of I/O you will have both a input (ex.I:19.00) and output (ex.O:19.00) for the same slot number.
I have quite a few scale heads on remote I/O and I read the scale value on the input word and use the output word for scale comands.
just thought it worth metioning.
 
Wire labeling

The usual (and useful) format for wire labeling where I work is:

cpXX I:02/03


Processor name,then bit or word identification. Makes it easier 10 years down the road to trace things.
 

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