Introduction to PLC programming with OpenPLC, the first fully open source Programmable Logic Controller on the Raspberry Pi, and Modbus examples with Arduino Uno and ESP8266 PLC programming is very common in industry and home automation. This book describes how the Raspberry Pi 4 can be used as a Programmable Logic Controller. Before taking you into the programming, the author starts with the software installation on the Raspberry Pi and the PLC editor on the PC, followed by a description of the hardware. You'll then find interesting examples in the different programming languages complying with the IEC 61131-3 standard. This manual also explains in detail how to use the PLC editor and how to load and execute the programs on the Raspberry Pi. All IEC languages are explained with examples, starting with LD (Ladder Diagram) over ST (Structured Control Language) to SFC (Special Function Chart). All examples can be downloaded from the author's website. Networking gets thorough attention too. The Arduino Uno and the ESP8266 are programmed as ModbusRTU or ModbusTCP modules to get access to external peripherals, reading sensors and switching electrical loads. I/O circuits complying with the 24 V industry standard may also be of interest for the reader. The book ends with an overview of commands for ST and LD. After reading the book, the reader will be able to create his own controllers with the Raspberry Pi.