Finally, here are the sources I was talking about for so long! This project page wouldn’t be complete without it, would it? The zip file below contains complete sources of AVR firmware in Arduino language (C++ with a couple of libraries, basically), the graphical development environment, a sample control program with corresponding configuration, a simple PC SCADA application and a simple communication library in Java. plc-project-061010.zip Schematics and other info can be found here (just search in the document, you’ll find it at the end). It looks like that: Do read more about it here.