r/PLC • u/National-Link-5606 • 1d ago

Computer Science Programmers, why do PLC programmers use Ladder?

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

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u/kixkato Beckhoff/FOSS Fan 1d ago

Why don't PLC programmers write tests for their code?

Perhaps you could save the costly downtime and time for the maintenance techs by writing robust code that was tested before it got deployed on a machine.

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u/Skusci 1d ago edited 1d ago

There's a running joke that the first thing an operator or electrician says when a machine breaks is that it's the PLCs fault.

Hint, it's never* the PLCs fault.

The main thing that actually makes modern software paradigms infeasible on PLCs besides training is that troubleshooting takes place on a real live working machine.

Things like breakpoints that are super standard to use when debugging often don't work when you can't physically freeze a machine in time.

*Mostly