You said it right, Charles.

I also have no qualms in using these tiny beasts and have in fact seen the ones from Siemens that Russell mentioned and used similiar ones from Mitsubishi - accepted they are cute and simple to get going.

But when it comes to analog I/O and user interactive diagnostics and informative displays they do lack shine. With a simple vanilla 8031 + 16Cx2L LCD, I can come up with the most user friendly "PLC" that interacts with the user in simple grammatically correct english. And that is only the begining !

But the advantage with such PLCs is the hardware abstraction that is achived - the user does not have to know what a gate is and still can use it very successfully.

Like everything else in life, each ( PLC vs MCU ) has its downside - which side to take is up to you.

Raghu