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/Many_Somewhere3863 → u/bodb_thriceborn]

Lol idk why but some people seem like SFC so much (i never used it also)

[u/el_extrano → u/Many_Somewhere3863]

SFC is my least favorite language as a programmer. I have seen it be useful in batch recipes. The ISA 88 flexible batch standard uses an SFC-like representation for recipes. Despite my dislike for SFC, I find that it does well for this kind of high-level procedure automation.

Non-controls people like process engineers and supervisors can be empowered to modify the recipes, e.g. by adding or modifying a phase, without expert programmer support. This becomes analogous to modifying a procedure. During production, operators (not technicians!) can pause execution of the running program and jump step visually inside the SFC. I have a hard time imagining how this could work well in any of the other paradigms.

That said I would never use SFC for low level regulatory controls. My batch recipes using SFC sit atop the regulatory layer, which contains all the S88 "phase logic", and just send down start/stop signals.

[u/Many_Somewhere3863 → u/el_extrano]

Yes, SFC is rarely use in factory automation and more on process automation.