PLC

Hi guise, I want to write a PLC simulator, any ideas on existing opensource projects to get me started?
  I've seen Karnaugh maps solvers on codeproject, since they may prove useful, but I can't seem to find much stuff.

Programmable logic controller, I would interpret the code from a ladder logic diagram.

And checking out SICP, but I'm actually planning on doing this in C/C++ or at the very least C#.