Modbus

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Modbus Protocol

Modbus is a data communication protocol that is based on a request-response model. Previously referred to as a master-slave protocol, the Modbus organization replaced the terms master and slave with server and client in 2020.

Modbus is used for transmitting information between devices that are connected to buses or networks over serial lines or Ethernet and, increasingly, using wireless.

Modbus is the most widely used network protocol in the industrial manufacturing sector. The most common use case is the communication between a human-machine interface (HMI) or supervisory control and data acquisition (SCADA) system and a sensor, programmable logic controller (PLC), or programmable automation controller (PAC). It provides a hardware-agnostic, cost-effective way to enable the interoperability of disparate automation equipment.

Modbus is popular because it is open source, simple to use, and used everywhere, making it a reliable solution for transferring discrete or analog I/O and register data between control devices.

Modbus is an application-layer messaging protocol, positioned at level 7 of the OSI model. The default port is 502 on a Modbus server device.

The first Modbus protocol – Modbus RTU (Remote Terminal Unit) – was originally published by Modicon (now Schneider Electric) systems in 1979 as a programming protocol for use with its PLCs. Modbus is an open protocol but the word "Modbus” is a registered trademark of Schneider Electric.

Modbus RTU is a simple serial communication protocol. Over time, there was a growing need for a standard that allowed more complex implementations using popular transport protocols like Transmission Control Protocol/Internet Protocol (TCP/IP) and User Datagram Protocol (UDP). Developed in 1999, the Modbus variant addressed this need.