Book Rags' PLC Basics
This is a wiki-based general introduction to PLCs that contains dozens of links to additional resources. It includes a comparison of PLCs and other controller systems, and chapters on programming, history, communications, interfaces and more. In addition, the site contains external links to a Linux PLC directory, a Power Point on ladder logic, and an interview with Dick Morley. The direct link is at www.bookrags.com/wiki/Programmable_logic_controller.