Different plc languages

[Liam 28]

Hi All

I was at a interview for work experience for college and I was asked about different methods of programming plc I said I was familiar with ladder logic and SFC . And I was asked could program through words say text something similiar to c code . I was not aware of this could any one tell me a good book or site where I could read up and research this method also any thing good on DCS and modbus.Any help would be great.

 

[widelto]

Go to www.ab.com and download following publication:
Publication 1756-PM007B-EN-P - July 2008
Logix 5000 controllers structured text

 

[Liam 28]

Thanks for the link. This may sound silly but the code I was looking at in the document does this allow you to to control say a output which may start a motor. Can you write a complete program in this langugae or is it used in conjuction with ladder if so would ladder be on one page and then this text on another or are they both on the same page of code. Sorry for all these questions but is there a name for this type of programming like there is for ladder or sfc.Thanks again for the help.

 

[KP_EENG]

Try this website: http://www.3s-software.com/index.shtml?en_CoDeSysV3_en

You should be able to download the free CoDeSys programming software which will allow you to program in Structured Text and test it in the emulation mode. I use version V2.3 with Wago PFCs.

Regards,

 

[Liam 28]

Thanks for the link appreicate it.

 

[L D[AR2P#0.0]]

Here's a very recent example involving motors.

http://www.plctalk.net/qanda/showthread.php?t=51397

 

[twu026]

Yes, you can turn ON/OFF real outputs using Structured Text (ST).

Output := TRUE; (equivalent to a SET coil/instruction)

Output := FALSE; (equivalent to a RESET coil/instruction)

Q := (A AND B) OR C;

--|A|--|B|----------(Q)
|............|
.----|C|----

Do some background reading into Boolean algebra if you are not already familiar with the topic. It should help you express logic in ST.

You can also write a whole program in ST if you wish. I generally won't recommended it. (But if you have to program a Beckhoff PLC then...)

I tend to only use ST for data manipulation. If you want to perform operations on arrays, UDTs or strings etc it is generally easier and clearer to do so in ST.
It would also help if you learn a language like C or (C++ or Java to a lesser extent).

As an aside, I make a rule of writing to real outputs only in one location and only in ladder. This speeds up troubleshooting.

 

[Liam 28]

Thanks guys for the help really appreciate it it gives me a starting point to reearch structred text.I was wondering if the structered text is combined with ladder say on the say page or is there different files say one for ladder and one for structured text and would be downloaded to the PLC as aseperate files.

 

[widelto]

As separated files, at least on AB.

 

[KP_EENG]

With all of the PLCs that I use (AB, Wago, GE RX3i) you are forced to create a different file for each type of code. They cannot be used in the same file, but you can call another file or program block written in a different language. Typically, I will write code that is math intensive in ST but leave things like interlocks and standard logic in ladder.

Regards

 

[timjager]

I agree with KP_EENG. Modularize your code into manageable function blocks and choose the language for each block based upon the task at hand. Here is a simple blog that I wrote on this topic: http://www.dmcinfo.com/Blog/articleType/ArticleView/articleId/111/IEC-61131-3-Choosing-a-Programming-Language.aspx

 

[Liam 28]

Hi Tim thanks for the link to your blog I found it a great read and very informitive.

Liam