The Modbus protocol is a communication protocol that allows devices to communicate over various types of media, such as serial lines and ethernet. It was developed in 1979 by Modicon, a company that produced Programmable Logic Controllers (PLCs), to enable these devices to communicate with each other.
Modbus provides a messaging structure designed to establish master-slave communication between intelligent devices. A Modbus message sent from a Device A (master) will initiate a response from Device B (slave). The function of the Modbus protocol is to define the content of the communication, how the information is packaged, and the order in which messages are sent and received.