Eric,

I develop industrial machines for the food industry. I use both PLCs and PICs. I will attempt to answer your questions / observation.

I believe that you aren't comparing apples to apples. Heck I don't think that you are comparing apples to oranges, more like apples to apple seeds!

First a PLC is an industrial grade computer. As you probably know you can supply a PLC with power (usually 80-240VAC) and it will accept inputs, NPN or PNP, and it has outputs, relay, NPN, or PNP. There are of course other options. Some PLCs provide 24Vdc out for sensors, most have some form of comunication protocol. Some even offer onboard ethernet. PLCs are usually very rugged by design. The inputs and outputs are often protected and the unit is not very susceptible to electrical noise or interference. PLCs come in a variety of sizes and features and range greatly in price. A simple PLC might cost $100 and a large unit might cost over $5000. PLCs are "off the shelf" solutions and do a variety of jobs.

A PIC on the other hand is a microcontroller. A PIC would need a suitable circuit built around it to work like a PLC. Because it runs off of a low DC voltage, usually around 5VDC and it has inputs and outputs which are usually TTL (some pins are open collector). You can't hook up most sensors directly to a PIC. You usually can't drive a load over 20mA per pin. PICs are rugged for a micro but not compared to a PLC and care must be taken in building the circuit.

You must design the electronics and there is a cost involved in designing the electronics around the PIC. Then you can write your code and debug it. With a PLC you simply buy it and program it. I use PLCs for one time projects or when I do not want to maintain the electronics. I use PICs when the project is large and there will be a cost savings or when I want sell the electronics.

I have never found a project that a PIC was too slow for but a PLC wasn't. It (usually) costs more to build a single PIC based project then it does to build a PLC based project. This is because of the development costs to design the circuit. The savings in using a PIC and building your own circuit comes when you will sell many devices. For me the trick was in desinging a circuit that could replace a small PLC. This allows me to spread the development costs over several projects.

Regards,

Joe Kupcha