Basic question

Hello all,
I am new to PLCs and have one question. There are many PLCs having different processors/controllers but the PLC is programmed using ladder diagram. When the ladder diagram is converted and loaded into a PLC, is it loaded in some form of an intermediate code which all PLCs understand or is it converted to specific processor/PLC? If there is a generic (intermediate) code what is this code called and how does it look like? Thank you.

Partha

there are other programing methods than ladder like statment list prety much each brand plc has its own software and a lot of brands have diff. software for diff. models

The software for PLCs generally is brand specific. And often, there are different software for different models by the same manufacturer. I'm sure when the code is converted from ladder, it is different from all others. Just because it's ladder, doesn't make it universal. Each manufacturer has his own dialect.

No.
The code in the PLC is brand specific.
The ladder you see is is merely a representation of said code.

Hello all,
Thanks a lot for your replies. So, for the same ladder the code generated for different PLCs is different. So if one writes a ladder and "compiles" it for one PLC, and the ladder needs to be compiled again if another PLC is used!! Thank you very much for clearing my doubt

partha_nayani said:

Hello all,
Thanks a lot for your replies. So, for the same ladder the code generated for different PLCs is different. So if one writes a ladder and "compiles" it for one PLC, and the ladder needs to be compiled again if another PLC is used!! Thank you very much for clearing my doubt

Click to expand...

Not only you need to compile it again, you most likely need to write it again with another programming software.

Hi TurpoUrpo,
But why should one write in another programming software? I thought all PLCs can be programmed using ladder diagrams!!! What other languages do PLCs in general support? Please enlighten me. Thanks

Because the "ladder" is different between different brand of PLC, not to mention different brand of PLC support different instructions.

Typical language for PLCs: http://en.wikipedia.org/wiki/IEC_61131-3

While one might think Ladder is Ladder, this not the case. Each PLC Manufacturer has their own Instruction Set. This is not the same as programming in C, C++ etc. Yes each PLC has an On Timer Instruction, but they are implemented differently. There may or may not be an Off Timer Instruction. As well, the Time Base for Timers vary by Manufacturer. Also, the format can be different, some have Vertical Shorts, others don't etc. There are many other differences.

So, you can't just Copy and Paste Ladder Logic from one PLC to another.

Stu....

partha_nayani said:

Hi TurpoUrpo,
But why should one write in another programming software? I thought all PLCs can be programmed using ladder diagrams!!! What other languages do PLCs in general support? Please enlighten me. Thanks

Click to expand...

Think of it this way: You can't run Apple specific programs on a Windows platform. You can't directly run Windows programs on a Linux operting system. Each PLC manufacturer has their own operating system and their own firmware resident in the PLC processor. Ladder logic is simply a graphical representation of the program intended to emulate electrical schematics. Whether the programming is done in ladder or sequential function chart or other language it is compiled or interpreted in the process of downloading to the PLC. The PLC executes this Boolean machine language, which is similar to assembly language on a PC.

Manufacturers tend to keep their compilers and their programming environment proprietary and unique in order to maintain a competitive edge in the market.

Hi Tom Jenkins,
Thanks for the update. Understood the need to re-write the program (may be modify the ladder to suit manufacturer specific converter. By the way what are these converters called?). So even downloading methods may be different from manufacturer to manufacturer.

Yo have the general idea. Maybe half of the questions on these forums are from people who understand how to perform programming with one company's product but are confused when trying to do the same with another company's products, or even between different software packages by the same company.

Hi Bernie,
Thank you. So when we buy a PLC from a company we need to get their programming tool and learn to use it. Next time you buy another model from the same company you may have to get another programming tool from them and learn this again (though it may be pretty easy). So I guess downloading tools may also differ from model to model!!!

Thank you. So when we buy a PLC from a company we need to get their programming tool and learn to use it. Next time you buy another model from the same company you may have to get another programming tool from them and learn this again (though it may be pretty easy). So I guess downloading tools may also differ from model to model!!!

Click to expand...

Yes and no. If you are using an AB SLC500 and then an AB Control Logix, for example, they use different software. I think the Micros also use different software although I have never used the Micros.
This same situation exists with all brands really. Certainly with Siemens, Schneider etcetera.
There is also very little done in the way of supporting old products with new software.
Additionally the newer PLCs run on a different hardware platform to the old ones.
They also usually have different function sets and some have separate maths processors in them where the old ones did not.
The only partial exception I have seen in recent years is Omron. The solution is not complete but better than most.
You can use the same software to program the shoe box, rackless mid range and rack type high end PLCs. You can also use the same software to program the fully redundant PLCs. You still need different software for the relay replacement type units.
The current software will not program any of the older PLCs but will convert an old program into something usable, usually with hand modifications, to use in one of the new PLCs. This type of support goes back to the old flat packs available in the late eighties and early nineties. 1990 model rack type PLCs are fully supported in the latest software.
The there are screens and different software for different model screens from the same manufacturer.
It is endless!!!
You will really need to check what software is required to program which model PLC from your manufacturer.

partha_nayani said:

...If there is a generic (intermediate) code what is this code called and how does it look like? Thank you.
Partha

Click to expand...

Ladder is converted to Boolean Logic. In Boolean, an AND gate is AxB=C. In Ladder, it would be drawn as a rung with 2 inputs in series with an output. A PLC uses numbers instead of letters, because it references addresses. If a PLC uses 1-99 for inputs, and 100-200 for outputs, the PLC software would show the Boolean as:
LOD 1
AND 2
OUT 100

What everyone else said is true. You still need specific brand software. They all start out with microprocessors, but there isn't a standard operating system like DOS or Windows. Everyone has their own operating system. Ladder diagrams look similar, and Boolean looks similar, but the similarities end there.