Here it is! The AT90USB646-based homebrew programmable logic controller. It’s not quite a Simatic S7 (and it’s not even completed in most parts), but it’s a nice start. A more complete and IEC-ish environment on the development software part (among many other things) would be nicer, but… the code is (will be) here, so extend it to whatever you want. But, whatever you may want to do with it, keep in mind it’s more of a “proof-of-concept” thing than a production-ready device.