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.

Thanks guys. I was browsing SICP and it is Scheme oriented, would it be too much of a hassle to write c++ code based on this?

Also, >>5 are you referring to the chapter on modeling with mutable variables (3.3)?