Yes, I have developed a AT89S8252 PLC with 16 inputs and 16 outputs, which can simulate most of general PLC function i.e. Logical AND,OR,Timer,Counter,Pulse, MOV instruction ... etc. Also a VB program to download code to MCU using ISP.
If you are interested, let me know.

EXACTLY the wrong approach, If you want a PLC, use a PLC. The lowcost PLC units (e.g. Keyence) perform and have excellent supporting software which nonbody that is not making thousands of units a year can afford to develop.

A uC should ONLY be used as a PLC replacement when the application can be coded in 'uC language'.


Erik