Modbus Protocol

Modbus is a networking system for industrial sites.

Modbus is an open standard for connecting sensors and data-gathering equipment (Remote Terminal Units) to a central reporting tool. It also sends commands to Programmable Logic Controllers (PLCs) to direct industrial machinery. It is particularly popular in industrial applications. The protocol specifies a series of message formats for communicating status information.

History

Modicon (now a subsidiary of Schneider Quantum PLC) developed a Modular Digital Controller (MODICON) and then developed Modbus in 1979 to define messaging standards, enabling this PLC to communicate with the engineering office. The protection and development of the standard is promoted by the Modbus Organization, which is an association of users and producers of Modbus-compatible products.

Versions

Five different versions of Modbus are available, and all are current. Modbus RTU, Modbus ASCII, Modbus TCP/IP and Modbus over TCP/IP are open standards that are freely available. The fifth version, Modbus Plus, is copyrighted to Shneider Quantum PLC.

Features

Each node on the Modbus network is assigned a number for identification. The Modbus message contains the destination ID and a message type. Modbus does not allow for variables to be added to a message.

References

• Real Time Automation: Modbus RTU
• Omega Engineering: Modbus RTU Protocol Communications Guide
• Modbus Organization: About Modbus Organization
• Triangle Microworks: Modbus Highlights
• Modbus: Introduction to PLC and SCADA