Introduction to Modbus Protocol


ModBus was originally developed by MODICON, now a part of Schneider Automation. The protocol has been widely utilized, with some slight adaptations by other companies.

There is a significant installed base in the USA as well as Europe, and many DCS (Distributed Control Systems) companies use ModBus as a communication protocol to their systems.

The ModBus protocol provides the internal standard that the MODICON controllers use for parsing messages.

During communications on a ModBus network, the protocol determines how each controller will know its device address, recognize a message addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message. If a reply is required, the controller will construct the reply message and send it using ModBus protocol.

The Modbus Protocol

Modicon originally developed the Modbus protocol in 1978 to exchange information between products on the factory floor. This protocol became a de facto standard for exchanging data and communication information between PLC systems.

A relatively simple protocol, Modbus has been implemented by many manufacturers of instrumentation and control equipment to offer system interoperability. Equipment supporting Modbus protocol variants include PLC’s, RTU’s, VFD’s (Variable Frequency Drives), SCADA Hosts, MMI’s, Flow Computers, Power Meters, Power Line Reclosers, Valve Actuators, Intelligent Instruments, and Protocol Converters.

The MODBUS standard defines an application layer messaging protocol, positioned at level 7 of the OSI model that provides “client/server” communications between devices connected on different types of buses or networks. It standardizes also a specific protocol on serial line to exchange MODBUS request between a master and one or several slaves.

Modicon programmable controllers can communicate with each other and with other devices over a variety of networks. Supported networks include the Modicon Modbus and Modbus Plus industrial networks, and standard networks such as MAP and Ethernet.

Networks are accessed by built-in ports in the controllers or by network adapters, option modules, and gateways that are available from Modicon. For original equipment manufacturers, Modicon ModConnect partner programs are available for closely integrating networks like Modbus Plus into proprietary product designs.

Supported by a wide range of manufacturers, Modbus protocol has been the protocol of choice when a single protocol is utilized in a SCADA communications network. A majority of industrial equipment either supports Modbus directly as a native protocol or via the manufacturers or a third party communication cards.

More recently Modbus TCP developed by Modicon has been adopted as industrial Ethernet protocol transporting Modbus protocol over LAN networks.

Enhancements to Modbus include Modbus Plus and Modbus/TCP protocols, both of which allow Modbus information to be encapsulated in a network structure to support peer-to-peer communications. Modbus Plus communicates via a single twisted pair of wires and uses a token passing sequence for peer communication sequences.

Modbus/TCP is an open standard designed to facilitate Modbus message transfer using TCP/IP protocol and standard Ethernet networks.