Actually you can write programs for running on the microcontroller to capture virtually any logical control alogorithm that you desire. There are limitations of course. Your program cannot exceed the size of the memory available to hold it. The data variables, state flags, and so forth cannot exceed the size of the read/writable RAM available, and for any given processor there is a limit to upper end of how fast you can make an algorithm loop.

I would say that if you are comfortable with getting started programming with a microcontroller I would jump right in and make the PLC a thing of the past. Virtually any custom microcontroller solution to a problem is going to have significant advantages over the PLC approach. A big one of those advantages is the cost of the equipment. PLC manufacturers like to convince industrial customers that you should pay them HUGH $$$ for PLC equipment that generally cannot out perform an 8051 assembly language, C language, or even other high level language implementation on native 8051 style hardware.

Michael Karas