How can I implement a SoftPLC?

I just want to implement a soft PLC,which supports Ladder Logic and Instruction List.But I don't know how to implement the interpreter of instruction list.In fact,the difficulity lies in the memory layout and organization of the interpreter.I think a interpreter just like a virtual machine,it has its own instruction sets,but I am lack of these knowledge.
Can someone give me some information?
Thanks for any help.

Bilbo

I am a little confused. To me "implement" implies that you are going to configure logic in a soft PLC program that you have purchased. However, your questions would indicate you are trying to write your own soft PLC program, since questions of memory organization etc. aren't usually important with purchased software. Please clarify.

If you are into creating your own softplc, then maybe you should team up with somebody that are allready working on it:

The MAT project

It is all linux and GNU. They could probably use an extra hand

bilbo0214 said:

I just want to implement a soft PLC,which supports Ladder Logic and Instruction List.But I don't know how to implement the interpreter of instruction list.In fact,the difficulity lies in the memory layout and organization of the interpreter.I think a interpreter just like a virtual machine,it has its own instruction sets,but I am lack of these knowledge.
Can someone give me some information?
Thanks for any help.

Bilbo

Click to expand...

If you can't do the interpreter then give up now. The interpreter is the easy part. Generating code for the interpeter is much harder. It is also MUCH hard to test since the combinations are endless.
Then there is the IDE. This is the 'face' of the PLC. This must be done well.

Maybe the word of "implemention" is not suit.
I just want to write my own soft PLC,I design the architecture of my soft PLC as below:
|----------| |--------------------| LD file
|IL editor | | Ladder Logic editor|------------
|----------| |--------------------| |
|IL text file |
| |------------| IL text |----------------|
|---->| IL compiler| <-------------| LDToILConverter|
|------------| file |----------------|
|
|IL object file
|--------------|
| Interpreter |
|--------------|
|
|
|-----------------|
| Virtual Machine |
|-----------------|

the diagram is not displayed correctly.
The aim of writing my own soft PLC is to test LD program or IL program
without PLC hardware.
I search for the information of soft PLC in the internet.But most of them only have specifications,no real implemention was explained.

The problems I faced were a lot,such as:
how to organize memory?
how to decide the sizes of IL instruction set ?(there are many instructions in IEC61131-3,do I need to implement them all?)
what is the format of input file of interpreter?(I don't want to interpret the IL text file directly,is this decison right?)
how to design the instruction set of virtual machine?(I mean if the instructions of VM is corresponding to IL instructions one to one,or more general,like JVM instruction set? )

I have no experience in these fields.
Thanks for any help.

bilbo

Bilbo,

I think JesperMP has the right idea, these guys have been working on one for quite some time.
Even if you don't want to join them, you can see what they have done and how they have done it. You can also download their source code.

My advice, learn from them.

Doug

OK, our posts crossed over.

For a quick, free program that allows you to simulate PLC operation, try:
http://www.tri-plc.com/

If you have some money available, I found the simulator for the Modicon (Schneider) Concept PLC programming software to be excellent. Also it implements all IEC PLC coding specifications (Ladder, FBD, ST, IL and SFC).

There are also others available.

You do not have to write your own unless you really really want to.

Doug

Why don't you download Beckhoff Twincat. It will run as a demo for 30 days each time you install it. You can run your programs on ikt, it supports all PLC languages, has simulated NC axis capability etc.

It is a very powerful piece of kit, does not cost the earth, programs in all languages at no extra cost ( ahem-AB) and all PLC code is completely transportable. That is to say hardware configuration and PLC code are completely separate from each other.