This timeline of the development of the PLC by different manufacturers is a work in progress. If you have something to contribute or correct especially concerning Siemens or Mitsubishi then please contact me.

PLCdev's article on The Birth of the PLC gives more information on how programmable logic controllers first got started.

Resources for this time line:

• You always have to start with Richard Morley's dusting off of his memory cells to tell us about the History of the PLC.
• Both Richard Morley and Allen Bradley's Odo Struger were chosen as one of InTech's 50 most influential industry innovators.
• Modicon to Gould to Schneider, Square D, Telemecanique
• Schneider Electric's complete history entitled From Modest Beginnings to World Leader, 163 years in review also in a nice pdf format.
• Original text from the Schneider Electric website listing Modicon's history.
• New Zealand has saved this little treasure.
• Allen Bradley to Rockwell Automation history
• Another good first hand account is Dr. Ernst Dummermuth's recollections in A Bit Twiddler's History of the PLC in the November 2002 A-B Journal.
• The Evolution of the PLC-5/1771 Controller System has a twisted path of development.
  
• Allen-Bradley: A Century of Quality covers the whole history of Rockwell Automation.  The A-B Journal has a different layout on their timeline.
• There's an excellent timeline in the article PLC 25 Years: Rockwell Automation traces the 25-year history of the Allen-Bradley PLC.  
• Omron PLCs
• Corporate History of Omron
• Automation Trends Newsletter of December 2002 Tracing Mid-Size PLC System Migration Over 15 Years
• General Electric Fanuc PLCs:
• GE Consumer & Industrial: A History of Innovation Timeline
• History of the GE Fanuc PLCs
• First hand experiences and memories
• History of Manufacturing, Industrial, Factory Automation Systems as we can remember it
• Jeremy Pollard has a good series in Control Design Magazine
• Genesis of the Programmable Logic Controller
• A PLC programmer’s pain -- 1980
• The saviors of PLC programmer sanity