#Reference : http://www.simplymodbus.ca/FAQ.htm
What is Modbus?
• Modbus is a serial communication protocol developed by Modicon published by Modicon® in 1979 for use with its programmable logic controllers (PLCs). • In simple terms, it is a method used for transmitting information over serial lines between electronic devices. • The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves. • In a standard Modbus network, there is one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. • The Master can also write information to the Slaves. • The official Modbus specification can be found at www.modbus.org/specs.php