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Modbus is a serial communication protocol published by Modicon in 1979. It is a de facto standard communication protocol in the industrial automation industry, used widely to connect electronic devices. Originally developed for communication between programmable logic controllers (PLCs), Modbus is now used to transmit information between a master (initiator) and one or more slaves (servers) over serial lines, such as RS-232 or RS-485, or via Ethernet.

The protocol defines a message structure that allows a device to request information from or send commands to another device. Modbus is a simple, open, and royalty-free protocol, making it widely adopted. It supports various data types, including coils (single bits), discrete inputs (single bits), input registers (16-bit integers), and holding registers (16-bit integers).

There are several variations of Modbus. Modbus RTU is a compact binary representation commonly used over serial lines. Modbus ASCII is a human-readable version also used over serial lines. Modbus TCP/IP is an adaptation of Modbus that runs over TCP/IP networks, typically Ethernet, allowing communication between devices on different networks.

Modbus operates on a master-slave or client-server architecture. The master device initiates all communication by sending a request to a specific slave device. The slave device responds to the master's request. This deterministic communication ensures reliability in industrial environments. Despite its age, Modbus remains a popular choice for SCADA systems, building automation, and other industrial control applications due to its simplicity and widespread support.