Allen Bradley PLC2 Ladder Logic

My mentor has me looking over some ladder logic from an old PLC program we have. Its apparently older then I am, and since the install we've lost almost all documentation to it (I've heard it might have been thrown out) thus leaving me more then slightly confused.

Looking at the program I seem to understand most of it. But occasionlly I find an output with a "g" in it. I know the L is latch, U is unlatch, and all the other symbols used, but I can't seem to come up with anything for it. It doesn't seem to make a lot of sense to be "ground" but I can't think of anything else.

Can anyone offer me some insight?
thanks

Golly, it's been a loooonnnngggg time...

Seems like it might be Get. Probably an analog value??

PUT & GET instuctions may be used as a set as well
GET data from, PUT data to, similar to a COP command in a way from what i've seen.
I don't think GET will be used to drive an output
GLOBAL maybe?

looking more into it, I've found an output with an = sign right next to it later in the program. Would the arithmetic compareter > be represented as a g? greater then? That might make sense as an ouput and something I hadn't noticed before it does have a value immediately below the output to compare it to.

take a look at page 5-9 ...

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1772-um004_-en-p.pdf

and put this book in your "Favorites" ... if your mentor keeps torturing you with this "oldie-but-goodie" stuff, you're going to need it ...

oh ... and welcome to the forum ...

wohoo, thank you and thank you!

Yes, ]G[ means Get but it would be inside of a contact, not an output. A (P) inside an output means Put. A Get/Put pair in a rung is the same as a Move (MOV) meaning get a value at a word address and put it in another word address. A Get instruction in a rung has no control over the false/true condition, it is always true. If you see two Gets they will be followed by an arithmetic instruction such as Add, Sub, etc. This means get the 2 values stored at the Get instruction addresses, do the arithmetic, and store the result at the arithmetic instruction address. There are also comparison instructions like ]<[, ]>[, ]=[ etc. which are used immediately after a Get to form a rung condition based on a number comparison. PLC-2 math can get really ugly if there is a lot of it.


I was a PLC-2 guru 25 years ago, let me know if you need any more help.

]G[ is a comparator in your case I guess
Upper XXX is the address.
Lower yyy is the actual data

In PLC2, ]<[ = comparator "less than", and ]=[ comparator "equal to".

To compare two values required first a "GET" to get the first value from memory, then the comparator instruction with the second value. Thus the statement "If the value in 020 (which is 005) is less than or equal to Timer 030, turn on Solenoid B" could be represented by:

 

|     020      030     Sol. B |

|-----]G[--+--]<[---+--( )----|

|      005 |        |         |

|          |   030  |         |

           +--]=[---+         |

A small point

steve708 said:

There are also comparison instructions like ]<[, ]>[, ]=[ etc. which are used immediately after a Get to form a rung condition based on a number comparison.

Click to expand...

As I remember there is no -]>[- (GREATER THAN) operator. There is only -]<[-. This is one of the troublesome things about PLC-2 math, one has to jiggle the operands to make things work.

We still have 5 models of the PLC2 still in service our plant.

If you do some Googling, there is a free PLC2 simulator available on the web. I think it is made by the same people who make LogixPro (a RSLogix 500 simulator).