Hi Richard,

Some time back, I had worked on a similar kind of conversion PLC to MCU. The main reason for this conversion was
1> The PLC code is programmed once in the Factory and never changed.
2> Cost Reduction

Its been close to 6 months now, its doing fine. But during the first initial phase we had few problems. You can read the whole interaction here -> http://www.8052.com/forum/read.phtml?id=56007

We also have completed a PLC using RD2 uC, It has 8 inputs , 8 outputs and 8 configurable I/O. The best part of this project was we designed a complier which converts PLC like language to 8051 assembly code. Though not so efficient can be used for simple applications.
http://www.8052.com/forum/read.phtml?id=54513
http://www.8052.com/forum/read.phtml?id=54521

We are planning to improvise the PLC...

Regards,
Raj Shetgar