MODBUS is a serial communication protocol published by Modicon in 1979 for use with its programmable logic controllers (PLCs). It has become a “de facto” standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices.

MODBUS allows the communication among many devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. MODBUS is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Versions of the MODBUS protocol exist for serial port and Ethernet. MODBUS/TCP transmits the protocol packets within TCP/IP data packets.

Each device intended to communicate using MODBUS is given a unique address. Any device can send out a MODBUS command, although usually only one master device does so. A MODBUS command contains the MODBUS address of the device it is intended for. Only the intended device will act on the command, even though other devices might receive it. All MODBUS commands contain checking information, ensuring that a command arrives undamaged. The basic MODBUS commands can instruct a RTU to change a value in one of its registers, as well as commanding the device to send back one or more values contained in its registers.

MODBUS is an application layer messaging protocol, positioned at level 7 of the OSI model, that provides client/server communication between devices connected on different types of buses or networks.

The industry’s serial “de facto” standard since 1979, MODBUS continues to enable millions of automation devices to communicate. Today, support for the simple and elegant structure of MODBUS continues to grow. The Internet community can access MODBUS at a reserved system port 502 on the TCP/IP stack.

MODBUS is a request/reply protocol and offers services specified by function codes. MODBUS function codes are elements of MODBUS request/reply PDUs.

MODBUS is currently implemented using:

• TCP/IP over Ethernet.
• Asynchronous serial transmission over a variety of media (wire : EIA/TIA-232-E, EIA-422, EIA/TIA-485-A; fiber, radio, etc.)
• MODBUS PLUS, a high speed token passing network.

as shown in the following figure.