SoftPLC - PC based controllers

I'm looking for documentation about anything that has to do with PLC which are PC based (possibilites, I/O, comparison to "classic" PLC, connectivity, solutions on market etc.). Since I lack of comprehensive knowledge about PLC, I don't have much idea what I'm actually looking for and Google did not bring and useful resources yet.

Could you point me out to the right direction?

Have a look at Codesys, the program is running on the PC, and the I/O is done with specialised PLC via any bus.

Rockwell Automation has SoftLogix controllers.

You can find out all you need to know about them in their website over here:
SoftLogix Control System