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Modbus Provides Simple, Low-Cost Ethernet Communication

Modbus is a venerable network protocol designed for communication of manufacturing data. It was introduced in 1976 as a proprietary way to send information to and from Modicon programmable logic controllers over a serial connection.

In 1996, Schneider Electric (www.schneiderelectric.com), which acquired Modicon, began promoting Ethernet as the best solution for industrial networking, calling the concept “Transparent Factory.” The Modbus protocol was just as easy to integrate with Ethernet as it was with serial networking. Thus was born Modbus TCP/IP.

In the meantime, control of the protocol was given to the independent Modbus Organization (www.modbus.org) and made freely available to any developers or users who wished to use it.

Ethernet defines Layers One and Two of the International Organization for Standardization’s (ISO) Open Systems Interconnect (OSI) model, which are the physical and primary connection layers. (See the sidebar for a description of the seven layers of the OSI standard.) Layer three is where the Internet Protocol (IP) resides. This is the Internet addressing scheme by which one device can find another device. Above that is the Transport Layer, which includes two different ways of transmitting information over Ethernet—Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

Modbus protocol resides at the top, or Application, layer. Other protocols at this layer contribute to the success of the Internet today. These include File Transfer Protocol (FTP), for transfer of large files, and HyperText Transfer Protocol (HTTP), for transmission of Web pages.

In practical terms, each protocol is embedded in the string of bits called the Ethernet frame. Devices that receive the information contain software that looks for certain bits in the string. If it sees the bits it likes, then it enables downloading of the entire message. Part of the success of the Internet is that FTP and HTTP are included with all Microsoft Windows-based personal computers and are easy to use. Unlike other industrial networks, Modbus does not require a special network chip to communicate. Each device only needs the Modbus driver with a standard personal computer.

Diego Areces, Ethernet marketing leader at North Andover, Mass.-based Schneider Electric Automation, says that Modbus today is primarily deployed in discrete manufacturing applications. “Because it does not require special chips and a lot of memory,” he states, “it only takes a few hours to integrate into a new application, rather than the several days that other solutions require.”

The most common applications are remote terminal units and remote substations, according to Areces. “It is used to move information around. A Modbus message of up to 125 [16-bit] words can not only read and write words or bits from the controller or input/output device, but it can also be used to download and upload information such as program changes to the controllers. It is the foundation of everything we use.”

Modbus TCP/IP is a master/ slave network. A device that is a master (also known as client) requests information from a slave (server). All that is required is a function block within the master’s program that sends the request and then receives the message via IP address. All devices that will communicate on the network must have an IP address.

The heart of Modbus is a suite of special function codes that defines specific operations. There are public codes that are used by everyone, and private codes for use within one company’s products. For example, public function code 03 allows the master to read words of data from the slave. The Modbus organization manages the public codes to assure interoperability of all Modbus devices on the network. “The Holy Grail of networking,” states Areces, “is a worldwide catalog of interoperable devices.”

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