What is Modbus?

Modbus was developed in the late 1970's by Modicon, which had invented the PLC (programmable logic controller) about 10 years earlier.  In 1979, Modbus published the first specification for Modbus, a communication interface for a multidrop network based on a master/client architecture.  The original Modbus interface ran on RS-232, but most later versions ran on RS-485 to accommodate longer physical distances between nodes, higher speeds, and implementation of a multi-drop network.  Its subsequent rapid acceptance by industrial vendors and users alike are a testament to its simplicity, flexibility, robustness, and ease of implementation.  Today the Modbus protocol has extended to wireless communications and TCP/IP networks.

Controllers using Modbus in its pure state communicate via a master/slave relationship.  In other words, one controller initiates a communication by sending out a message, and the other devices either perform a requested operation or send back requested data to the master.  The structure of messages is a 10-bit packet consisting of:

• Device Address
• Function Code
• 8-bit Data Bytes
• Error Checking

Because all messages have well-defined start and end points, it is easy for receiving devices to detect when message first arrives and when it has transmitted in it entirety.

Modbus Today

Perhaps the biggest testament to Modbus is its continued popularity and relevance more than 30 years after its inception.  The Modbus protocol has its own organization, Modbus.org, a group of independent users and suppliers of automation devices that seeks to drive the adoption of the Modbus communication protocol suite and the evolution to address architectures for distributed automation systems across multiple market segments.  Ken Crater, Control Technology Corp.'s Founder and Chairman, is also the the founding president of Modbus.org.