PLC I know... now whats SPS??? :-))

There's plenty big companies into PLCs... Apart from Siemens, there's Allen-Bradley (the most popular), Omron, and many more.

A PLC is a controller optimized for digital inputs and outputs, although I have worked on analog inputs too. I think the PLC was originally designed to replace mechanical relay-logic circuits in the automotive assembly lines, hence it is often programmed in terms of Ladder Diagrams. But when I worked on the Siemens Simatic S5 PLC, it offered an interface that allowed programming in 3 modes: OpCodes (STL), Ladder Diagrams, Digital Circuits. The STL was like very very limited assembly opcodes, but the more advanced systems allow structured programming, jumps, math statements and even PID controls.

It offers a less flexible, but simpler, ready-made, more reliable and more rugged alternative to programming your own uC from scratch. Also its modular design allow you to reprogram and install more input and output cards even after installation.

Mehmet,
I would be very interested in knowing how you did it all, as I had once planned to design one myself as my Final Year project ... I even designed a parser in Java for my own PLC language. Unfortunately I do not understand Turkish.

After the programmer has entered his program, what do you feed into the 8051? Do you provide some code that the 8051 program interprets at run-time, or do you generate the Hex program for the 8051 itself?

Kundi