STL vs. LADDER

SLaubach

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Join Date
Apr 2002
Location
PA
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152
I just got back from Siemens Programming I in Baltimore and i was wondering how common STL is in the US? The instructor who was VERY good kept jamming STL down our throuts. This was my fist exposure to Siemens (AB guy) and of course not knowing anything about siemens let alone statement lists i think I could have got more if he would have tought in ladder.
 
The good news is that you can use both

My S7 programs look like a mixture of both LAD and STL. Unfortunately Siemens was brain dead and forgot to make an easy indirect addressing instruction in LAD, so I use STL for accessing tables and doing anything complicated. Simple coil and relay stuff can be done in LAD. One can write the program in LAD and view it in STL. From this one can see that STL is MUCH more efficient than LAD.
If you use S7s, eventually you will learn STL. I don't know of any S7-300 programmers that never use STL.

Having the choice to use STL or LAD is a plus.
Being forced to use STL for indirect addressing is a big minus.

'You vill do it die Ziemens vay'.
 
STL vs LAD

From my limited experience as a PLC programmer with Siemens S7 i can tell you that I prefer STL, it´s much mor flexible when you have to develop complicated programs. LAD is easy to use end more didactical, but when it comes to a more complicated task I prefer STL.
In a Siemens program you can combine STL with LAD, but you can put them together in the same network of your block.

Gery
 
Hi lads,

as a PLC and SCADA trainer, I always advise my trainees to stick to LAD as much as possible. The reason is that this way the most people can read the program. Is the main ruke with PLC programming not KISS? (for those who do not know KISS: it means K eep It Simple, Stupid - sorry for those who are aware of this hopefully totally unneccesary explanation).

You can stretch this rule even further: there is a programming method called Graph7, which implements grafcet to the S7. I mostly develop my program examples (as I am a teacher) with grafcet, but I have NEVER used Graph7. I always translate it to ladder. And a good bonus with this method is that I remain able to implement the developed proces with electronic circuits, relay circuits and even with pneumatic circuits! Talk about fredom!

By using ladder it remains also fairly simple to convert to another brand of PLC. Try to translate S7 STL to AB or OMRON, and you'll see what I mean.

Kind regards,

Jean Pierre Vandecandelaere
Instructor PLC - SCADA
VDAB (Flemish Vocational Training Instance)
 
As a long time S5 programmer I don't find the indirect address thing much of a problem though I agree that it would be nice to have some LAD function for it.

What STL is great for is to write things like scale blocks in FC OR FB with parameters and then use them over and over. With Siemens stuff if you get good at writing function blocks you wind up with a library that sure saves time.

Then you do all the logic in ladder so folks can see the go's-indas and come's-outtas then nest your function blocks in the ladder for structure. Good by linear programs forever!
 
I think you should change LAD to STL by your PG before you add more STL. I think in menu of Step5 or Step7 there are this functions.
 
reaply

hi dear

you can go in the tool bar . click on view you can see lader or stl ,,, so after u write programe in lader u can go to view and click on stl .....
i hope this what do you want...

mohammed
 
I have used all the main languages on large projects, and my conclusion is:

1. Ladder is best for about 80% of the code because it is best at visualising the flow of logic within a statement.

2. Function Block is prefered when the code is more about the flow of the data than logic intensive. Usually best at continuous tasks associated with complex PID controls.

3. SFC (Sequential Function Chart) is not really a language, but a means of organising segments of code at a higher level. Useful in some applications, but not all.

4. ST (Structured Text) is there for just three reasons, one so that "prgramming geeks" who can only think in text based languages can do automation, two, some applications can have very intensive case-based decision making that is more efficient to do in ST, and finally, so that the European vendors can get away with selling half-a$$ed software with useless ladder editors.

There is no doubt that highly proficient programmers will tend to migrate towards text based programming because it is faster for them to type it in, but the downside is decreased readability for anyone else following them. ST is ok if your software is only ever going to be read by your fellow geeks, but automation code is often around for decades after it was written and may need accessing by many different people over that time, most of whom are just trying to do a little troubleshooting or a few mods.

For that reason I believe Ladder will remain the "universal" language for automation for a long time to come.
 
Last edited:
So if some people are stuck on the perceived clarity of LAD, and others value the flexibility of STL, and others despise ST (sorry Philip!), what's to stop you tackling it as follows -
- write any logic-heavy code in LAD and parcel it up in a block;
- write any really low-level wow-that's-tricky code in STL and parcel it up in a block;
- write a case structure for a geek in ST and parcel it up in a block.

And now here's the beauty. Call these blocks in LAD!
Everybody happy now?

Ken
 

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