Modbus is a client/server data communications protocol in the application layer of the OSI model. It is a method used for transmitting information over serial lines between electronic devices. Modbus is an open protocol, meaning that its free for manufacturers to build into their equipment without having to pay royalties. It was originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs) and has become a de facto standard communication protocol. Modbus is popular in industrial environments because it is openly published and royalty-free, developed for industrial applications, is relatively easy to deploy and maintain compared to other standards, and places few restrictions on the format of the data to be transmitted.

In a standard Modbus network, there is one client and up to 247 servers, each with a unique server address from 1 to 247. The client can also write information to the servers. Modbus is often used to connect a plant/system supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Modbus is typically used to transmit signals from instrumentation and control devices back to a main controller or data gathering system.

The Modbus standard supplies a relatively simplistic data model that does not include additional data types outside of an unsigned word and bit value. Although this is sufficient for some systems, it is insufficient for more advanced systems. As a result, many Modbus implementations include data types that cross register boundaries. Modbus can run on many network layers besides serial and TCP.

Each Modbus message has the same structure, and four basic elements are present in each message. The sequence and order of these elements are the same for all messages, which allows for easy parsing of the content of the Modbus message. A conversation is always started by a master in the Modbus network. A Modbus master sends a message, and depending on the contents of the message, the slave interprets the message and responds to it. Modbus protocol devices will typically include a register map outlining where the configuration, input and output data can be written and read from. The Modbus data model has a simple structure described in four basic data types: Discrete Inputs, Coils Outputs, Input Registers (Input Data), and Holding Registers (Output Data).