PLC Processor's Working Memory

Or the output power of an internal combustion engine by its cubic capacity or swept volume.

The UK did it for vehicle tax purposes many years ago!!!

1 Liter engine capacity was 10 HP.

(Note the missing "B"!!)

Now that is a real "Good Moaning" is it not?

https://en.wikipedia.org/wiki/Programmable_logic_controller

Some reference material to go over and digest. I guess the buzzword is "Modular"!

Read the spec sheet.

Not all questions can be answered by calculations.

Simple, unplug the CPU module, there is a specs plate usually on the sides - read

Most PLC manuals give an approximation of the amount of memory used per I/O. If it is not in your manual then contact the supplier manufacturer and they will have an answer.

But..... If you need to calculate how much memory you have left, you have not got enough. All software "grows" in size as parts are rewritten during testing and debugging. Running out of capacity at that point and having to upgrade your CPU is a pain. You need at least 20% spare after the program is written and preferably 50% spare. With that amount of spare capacity the I/O drain becomes insignificant.

Working memory and I/O layout are independent quantities, as you would have learned had you not fallen asleep in class. If you remembered that PLC's are special-purpose computers, which you did at some level, since you used the term cpu, you'd remember that working memory, storage memory, and I/O are all separate features that can be designed larger or smaller as the computer designer sees fit.

You are also overlooking something you'll be learning about later in class: that counters, timers, and registers take up differing amounts of memory, so the 'size' of the program, in rungs, that you can write depends on how 'complex' the program is.

Also, different PLCs handle timers/counters/etc differently. Some give you a fixed number that are always available (and always using up memory space) whether you use them all or not, others allow you to 'create' timers and counters, *IF* you properly 'partition' the memory space first. and some will led you seemingly do what you want, because the software on the desktop computer is taking your program and 'compiling it down' into PLC instructions when you 'download' the program to the PLC.

Good luck in your studies, and don't be afraid to ask your teachers questions; they'll be happy to answer them, and they will be less demeaning and snarky than this bunch of grumpy engineers, myself included, if you can find me.