Programmable logic controllers: Hardware, software architecture Part 1 from InTech magazine.
AllAboutCircuits.com Ladder Logic including PLC’s.
Automated Manufacturing Systems with PLC’s free ebook by Hugh Jack at Engineeronadisk.com.
Siemens Basics of PLC’s Online Course (or PDF files). Take the Course Online (you must create a SITRAIN LMS account for access)
or Alternate download site for course pdf files
Other courses in the series
PLCS.net Your Personal PLC Tutor Site.