In the course of my business I use PLC's and design microcontroller based equipment. As has been said before - a PLC is just a pre programmed microcontroller. In fact many of the new PLC's you can reflash the microcontroller with your own code if you don't like theirs. The manufacturers of PLC's spend a lot of money making sure their products are failrly bulletproof - when you write you micro code do you take into account various crash modes? For your input and output tables - do you check to see if they have been erroneously overwritten? I know I never used to until a defect caused all my outputs to activate! The other issue is one of certification - in most parts of the world you need UL,FCC,CE etc and these take a lot of work and money. You can get some very cheap PLC's nowadays that have built in comms, a/d and user interfaces (~200usd and less!) so if all you need is some input, outputs and a bit of logic, then the PLC choice is a no-brainer especially if you only need less than 200 units. Also consider the cost of the ancillaries like a power supply - many PLCs have this built in.

When I design a microprocessor device, most of my work is in making the device robust for an industrial environment and ensuring it complies with relevant standards. I've got more hope of a customer reprogramming a PLC than reflashing a micro when the software has a defect - and I've had a few!

Checkout www.twido.com for some low end PLC's. I've used a few of these and their more than capable for most jobs - and they have a standard microprocessor in them with 256k flash -that can be reflashed! These also use a open standard comms protocol (modbus) so they're easy to communicate with from a PC.

I doubt if anyone could design a microprocessor based unit from scratch that is certified and in a enclosure then write a reasonably complex control program at less cost and in less time than it would take to write a PLC program and load and test into a PLC - let alone taking into account documentation, testing, reliability etc. Having said that, I'm still employed to design microprocessor based devices!