Erik,

We havent defined what a PLC is yet, I guess things will be clearer when we define it.

You cannot compare a 8051, which is a device and a PLC which is a tool made out of discrete devices. So on can build a PLC using a 8051 microcontroller. The problem is that today's PLCs neet so many inputs and have to contol so many outputs thats especially need realtime considerations.

MCS-51 contollers have very sparce stack space which makes them a poor choice for multitasking applications. These controllers traditionally fall miserably short, although some high speed variations like the 2x clock Philips P89C51Rx2 or Dallas 89C420 or 80C390 and 80C400 offer as much as 25~27 times better performance (consider a 3 clock architecture which is 4 times as fast at the same clock, and clock speeds as high as 75 MHz) over the standard 8051.

If you have a problem that does not have too many considerations, you can easily build a PLC with one of these high speed controllers. For smaller applications choice of a microcontroller still is the designers preference.

We have built a tiny 89C1051 based controller board for this very purpose. Please see the docs here:http://www.logic-hi.com/download/...-flyer.pdf

This board can be used for small applications which do not much any headroom for enhancement. The programmable virtual machine makes it a suitable platform for tiny-PLC applications. We are developing a tiny-PLC ladder application for this board.

After all, who doesnt want a sufficiently fast PLC at under Rs.700 or $20 ??? Of course for potentially bigger projects, one has to resort to faster processors.

girish