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Modbus in Automation and Process Control

01 September 2007

Modbus is simple, inexpensive, and universal--and as a consequence, the most popular industrial protocol in use today.

The latest industrial control equipment may have a wireless, Ethernet, or fieldbus interface, but there's one thing they all share--Modbus.

Modbus runs over virtually all communication media, including twisted pair wires, wireless, fibre optics, Ethernet, telephone modems, mobile phones and microwave. This means that a Modbus connection can be established in a new or existing plant fairly easily. In fact, one growing application for Modbus is providing digital communications in older plants, using existing twisted pair wiring.

What is Modbus?
Modbus was developed by Modicon (now Schneider Electric) in the USA in 1979 as a means for communicating with many devices over a single twisted pair wire. It was adapted to run on RS-485 to gain
faster speed, longer distances and a true multi-drop network. Modbus quickly became a de facto standard in the automation industry, and Modicon released it to the public as a royalty free protocol.

Today, Modbus-IDA (www.modbus.org), continues to support the Modbus protocol worldwide.

Modbus is a 'master-slave' system. The master typically is a PLC (Programmable Logic Controller), PC, DCS (Distributed Control System) or RTU (Remote Terminal Unit). Modbus RTU slaves are often field
devices, all of which connect to the network in a multidrop configuration (see diagram). When a Modbus RTU master wants information from a device, the master sends a message that contains the device's address, data it wants, and a checksum for error detection. Every other device on the network sees the message, but only the device that is addressed responds.

Slave devices on Modbus networks cannot initiate communication; they can only respond. Some manufacturers are developing hybrid devices that act as Modbus slaves, but also have write capability, thus making them pseudomasters at times.

The three most common Modbus versions used today are

....Modbus ASCII
....Modbus RTU
....Modbus/TCP

All Modbus messages are sent in the same format. The only difference in the three Modbus types is how the messages are coded.

In Modbus ASCII, all messages are coded in hexadecimal, using 4-bit ASCII characters. For every byte of information, two communication bytes are needed, twice as many as with Modbus RTU or Modbus/TCP. Modbus ASCII is the slowest of the three protocols, but is suitable when telephone modem or radio links are used.

In Modbus RTU, data are coded in binary, and require only one communication byte per data byte. This is
ideal for use over RS-232 or multi-drop RS-485 networks, at speeds from 1,200 to 115 Kbaud. The most common speeds are 9,600 and 19,200 baud. Modbus RTU is the most widely used protocol.

Modbus/TCP is simply Modbus over Ethernet. Instead of using device addresses to communicate with slave
devices, IP addresses are used. With Modbus/TCP, the Modbus data are simply encapsulated inside a TCP/IP packet. Hence, any Ethernet network that supports TCP/ IP should immediately support Modbus/TCP.


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