Advantages of different IEC 61131-3 Languages

TL140

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So Ladder is easy to read graphically, most everything can be coded in ladder, and it represents wiring diagrams the closest.
SCL and ST are good for things like lookup tables and moving a lot of data at once repetitively.
FBD is good for visual logic.
Instruction list represents assembly language the best.
And lets pretend Sequential Function Chart doesn't exist.

What are some of the pros/cons to each of the languages and what are some applications where you find one language more useful than the other?
 
Ladder - I typically don't use this (not common in the DCS I work with mainly). I see the advantages of easy reading a single line. I use ladder when programming some safety systems.

ST - Use this a lot for either doing my own reusable function blocks (type circuits) or advanced math. Use excel to "generate" bulk code in ST which makes it quick and easy. Lots of people confuses ST with Siemens STL (which is statement list, IEC instruction list), a completely different animal.

FBD - This I use a lot for cascaded control loops, intelockings and whatnot. I think it makes it easy to visualize how things work together.

IL - I really don't see any usage for this in a modern system. But that may just be me? Hard to read/understand.

SFC - (even though it doesn't exist..) easy way to get a sequence going, you don't have to do lot's of "fluffy"-coding around to get it working. The systems I use have automatic visualization in the OS for SFC:s. Quite nice.
 
Why would you leave out the most powerful language for implementing state machines? Far beats any of the other languages for creating and troubleshooting state machines.

Just a personal dislike for me. I agree with you that it's wonderful for creating state machines.
 
I wouldn't recommend using ST/SCL for general logic as it is slow to monitor/troubleshoot but it is mandatory if you need to do anything algorithmic.
 
Personally, I think Ladder makes certain bits of logic extremely easy to read, but not necessarily others.
It's damn near impossible to beat Ladder when it comes to interlock conditions, for example, but start adding timers, block calls and calculations and it easily gets messy.

One thing to comment on Ladder too is how flexible the development environment is. Siemens Ladder editor lets you put rungs into inputs for blocks which certainly helps keeping the code neatly together, whilst other manufacturers require you to put this into a variable and then use the variable.

FBD excels when displaying the logic flow of a process as everything gets linked together, but if there is a bit of logic that you need to ensure happens one cycle later, it gets clunky to read.

ST is great for writing calculations and data crunching. I find that simple bits of logic are ok-ish (depending on how good the editor is), but generally, try to steer away from it unless it will be something very special... this being said, some editors allow you to use three languages in the same block of code, so you can make it look really neat too.

Instruction list, is powerful if you understand it, but it's definitely not easy to read unless it's a calculation.
 
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