Help with 1st. LL program please, sample code included.

[snaggletto]

Thanks for the info Pinworm21. Let me soak this one up a bit and I'll get back with 'intelligent' questions.

Let me ask this though, you have "air cyl out 1" as the output at then end of rung 1. You also have "air cyl out 1" in the sub branch but as a contact, or input. Is it normally allowable to have the same names of inputs and outputs, even if the actual memory registers are different? Why I'm asking is due to the free Entertron PLC programming software I have. It has an offline simulator, when creating labels for inputs and outputs etc... it complains about the name not being unique. Is this just something weird about Entertron's software or what?

 

[mellis]

Couple of points...

Symbol names must be unique because they are effectively an alias for the address. In programming software that supports symbols, you can usually type the symbol in place of the address. Obviously, one symbol can't point to two addresses.

The address associated with a coil instruction is *not* automatically an output address. Also, the address associated with a "contact" instruction is *not* neccessarily an input address. In the example you referred to, the "contact" really does have the same address as the "coil" and is this case it is a real output address.

Keep plugging, you'll get there.

 

[pinworm21]

snaggletto,

You cannot have duplicate labels at all in the Entertron software (Which is a good practice, that is also where my example was written in). What you see in my example is both the output coil and the outputs associated contact, they are both the same element (Therefore they share the same label). I am just using the output contact to latch itself on once it was activated. Please let me know if you need anything else.


Dave

 

[Lancie1]

snaggletto,

I think that you are having difficulty understanding that a coil Output can also have an Input contact with the same address and the same symbol name. Perhaps you should relate this to a physical Relay.

You said that you were familar with relay logic. Well, PLCs are designed to replace relays. Now if you have a relay, call it "R1", it has a coil called "R1". But does not it also have 2, 3, or more contacts that are also called "R1"? These relay contacts can be used in other rungs for many functions--same as the input contacts from any output on a PLC!!

True, on a relay, many times the contacts are labeled with the terminal numbers, or with "R1-1", "R1-2". Well, guess what? On a PLC, you have virtually unlimited contacts for any specific output coil. These contact bits all have the same internal address and are labeled with the same symbol, usually automatically once you type in the first one. That is one of the advantages of a PLC.

Another confusion factor is that you are trying to think about the Inputs LS1, LS2, and figure out what to do with them. THis is a common mistake made by PLC programmers, which is amazing to me, because if you were using relays, you HAVE to first think about the OUTPUTs. As others have stated, you need to think about WHICH Outputs you will need FIRST. You know that you need Output 1 and Output 2. But do you need other "intermediate, temporary" output states (usually referred to as "internal PLC outputs") such as bit Coils, Timers, or Counters?

A Short Method to Start Your First Program

1. Make a list of all your outputs, then insert program rungs, with blank lines or dummy bit inputs on the left and nothing but the output on the right side. At this stage you should have ONLY ONE rung for each output.

2. Now go to the first output rung (forget about all the other outputs for now), and think about all the input conditions that need to be considered to make THIS output work correctly. Insert those input contacts. If there are other conditions that are not controlled by physical inputs, then insert internal relay outputs and contacts for those.

3. Do step 2 for each output rung, and you are finished with your basic program. Try it out and make changes as needed.

 

[snaggletto]

Hello,
Thanks for all the help. I looked through a basic PLC manual, learned some symbols and dived in head first. Obviously I have some very important concepts that I have yet to learn. I'm in the process of trying to find a more comprehensive programming book/tutorial. You have to start with a solid foundation...

 

[CaseyK]

In the manufacters links under the Learn PLC's section, you will find many manufacturers that provide free downloading of their manuals, most which are excellant learning tools.

I specifically like GE's manuals, and have used them in training classes that I conducted.

There is even a few PLC books that are downloadable, but I am not remembering where off the top off my head.

Someone will probably know and post it.

regards.....casey

 

[Eric Nelson]

There is even a few PLC books that are downloadable, but I am not remembering where off the top off my head.

Someone will probably know and post it.

Yup... HERE'S one...

beerchug

-Eric