Basically, PLCs operate in a loop that scans the inputs, runs the ladder logic code, and writes the outputs. First you need to define your instruction set. What things will it do. XIC, XIO, timers, counters, latches, compare, move, etc. Then you need to decide how the 8051 code will work. Will the 8051 function as an interpreter, or will your relay ladder logic development software build a program file for the 8051. I imagine most PLCs function as interpreters. You could use internal program memory of the 8051 for the interpreter and then put the RLL code in an external FLASH memory. Modern PLC IDEs offer support for languages other than RLL, but RLL is a must. It seems to me an exceptional amount of trouble when off the shelf PLCs are so inexpensive.

Cory Spackman