embedded systems - modbus - message format - RTU
embedded systems - modbus - message format - TCP
embedded systems - modbus - registers
embedded systems - modbus - function codes
embedded systems - modbus - crc using table
embedded systems - modbus - crc calculation
embedded systems - modbus - exception codes

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Documents
Modicon Modbus Protocol Reference Guide
Modbus Application Protocol V1 1b

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References
Modbus Automation Client
PLC Simulator
Modbus article on Wikipedia
Simply Modbus
Modbus TCP/IP
http://freemodbus.berlios.de/ link dead on 09/26/2019
Sila FreeMODBUS
information on CRC calculation at lammertbies.nl
Modbus interface tutorial
What You should know about Modbus
Lintouch NG
Modbus Technical Resources
Detailed description of the Modbus TCP protocol with command examples

Modbus (RS-485) Using Arduino
How to Use Modbus with Arduino
modbus-esp8266
ESP-Modbus