A site that calls itself "The Most Unconventional Guide to PLCs on the Internet " http://www.plcdev.com/plc_basics
A history of Modicon/Schneider http://www.plcdev.com/schneider_electric_modicon_history
OPEN PLC
https://openplcproject.com/docs/openplc-overview/
https://openplcproject.com/download/
https://github.com/thiagoralves/OpenPLC_v3