Programmable logic controllers: Hardware, software architecture Part 1 from InTech magazine.
Programmable logic controller hardware Part 2.
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.
Recommended books:
Programmable Logic Controllers, 5th Ed, by Frank Petruzella, available from Amazon.
Introduction to Programmable Logic Controllers, 3rd Ed, by Gary Dunning, available from Amazon.
Electrical & Instrumentation Tech 