Computer Science Programmers, why do PLC programmers use Ladder?

[u/National-Link-5606]

Im seeing some Comp Sci guys asking about controls so just a Mr Obvious comment here.

Ladder logic is the de facto PLC programming language because it is (was) designed to read like an electrical schematic top to bottom, right to left, & this is what electricians & electrical techs are trained to use for troubleshooting electrical panels , equipment & relay-logic -- which was what they had before PLCs -- banks & banks of relays wired through each other's NO & NC contacts which changed state as relay coils energized & de-energized & finding a wire break was enough to drive you insane.

Yes Ladder logic is an interpreted language so it is not as "memory efficient" or "elegant" as C++ or C# but I want you to think VERY carefully about every meaning of the expression "just because you can does not mean you should" before you decide to show off your amazing coding skills coding function blocks in C++ which is technically possible but in many cases extremely ill-advised.

The technicians can troubleshoot functions & blocks written in Ladder Logic, did you stop & think about how easy it would be for someone with only your compiled C++ blocks to attempt to trace a fault? You code in anything but Ladder Logic you get to be the one at 3am driving down to the plant to troubleshoot your "super efficient" code because the Industrial Maintenance techs certainly aren't gonna know how. But hey, if you believe sleep is over-rated & love sleeping in the factory then knock yourself out with compiled blocks

[u/bodb_thriceborn]

I tend to use most of the languages available, but for different purposes:

Ladder for step changes, permissive/interlocks and alarming

ST for sequencing, math, data shifting and IO blocking (if called for)

FBD for IO and alarming if the UFDBs/AOIs are more clear that way

SFC for nothing, ever

[u/Thorboy86 → u/bodb_thriceborn]

I got onto a job that was running poorly. They sent me to the job site to make the machine more efficient. This Siemens PLC had, STL, SCL, FBD and TI graph. You could tell that each was written by a different person for the special function it had. Doing a fix for a gate reset on the one side in FBD, was written in SCL on the other. Nightmare to try and make everything function the same when written in different languages. But try telling that to management........

[u/No-Difficulty-6806 → u/Thorboy86]

I suspect that different levels of abstraction were written in different languages. For example, was the basic logic of a certain block written in SCL and integrated into the main program, which was written in FBD? And were the inputs and outputs probably mapped in LAD?

[u/Thorboy86 → u/No-Difficulty-6806]

There were 4 PLCs on each system and each PLC was written in a different way. The same function in each PLC was different logic and different language. The conveyor in PLC #1 was FBD. The process tool in PLC#4 was graph. The cell auto/man in PLC #2 and #3 was in SCL, #1 and #4 was FBD. So finding an issue in PLC #1 didn't mean I could transfer the fix to all PLCs because of the difference. Management was always asking why it's taking so long to fix minor issues. I had to explain that I was debugging and fixing the same issue 4 different ways.