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MODBUS: Evolution of Control Systems into Field-Bus Technology

Automation systems in recent years are making a transition from connecting to field devices using I/O signal modules to bus network communication systems called FIELDBUS. In this article, we would highlight the advantages of FIELDBUS control systems over the traditional voltage and current control methods, with MODBUS as the case study.

FIELDBUS SYSTEMS enables large amounts of data to be transmitted over single lines of communication (called a bus) that would require multiple signal wires using regular I/O modules. A bus is simply a collection of lines that transmit data and/or power. A case in point would be the control of a Variable Frequency Drive(VFD). A typical VFD has at least the following digital parameters: Start/Run, Stop, Forward, Reverse. This would require digital input and output signal modules and and several lengths of running wires. The analogue signal parameters are the frequency command from the PLC and frequency feedback. These would be connected the PLC analogue output and input cards respectively with reasonable lengths of running wires. For extra control or data acquisition even more wires are required.

A Fieldbus system such as MODBUS and other Fieldbus systems would be able to transmit all these parameters over a single line of communication cable. In addition to giving access to more parameter control/monitor and diagnostic data with no extra installation or wiring costs and hassles. This is made possible because the Controllers(PLCs) directly accesses the memory of the field devices to modify or read parameters in place of running control wires externally which does Digital to Analogue or Analogue to Digital conversions.

The Fieldbus communication protocol used for communication on the bus must be supported by both the PLC(s) and devices on the bus. This is because the PLC connects to and communicates with each field I/O device according to the bus’s protocol.

Each device on the bus has a unique address because the PLC uses an I/O bus network scanner which reads and writes to each field device using this address. Multiple devices can also be looped to create a network of fieldbus devices that communicate with each other building a more intelligent automation system. Devices can be added and removed from the network with very little hassle. Fieldbus cables can also transmit data over long distances making them suitable for use in remote I/O situations. There is also a variant called “intrinsically safe” which are used in hazardous environments.

There are different fieldbus variants available from different manufacturers, some of the most popular being PROFIBUS, MODBUS, INTERBUS, Foundation Fieldbus, DeviceNet, ControlNet, CanOpen and a host of others. Manufacturers are adding fieldbus capabilities to most of their new devices and are creating special modules for old devices without inherent fieldbus capability. We will take a closer look on MODBUS.


Modbus is the backbone of fieldbus communication protocols. It was originally created by Modicon (now Schneider Electric) in 1979. After its creation, it quickly became the first widely accepted Fieldbus standard.

Modbus is an open protocol which is accepted as a De-facto standard in the industry and is supported by many devices from different manufacturers. This makes it suitable for software integration of devices from different manufacturers as it has a standardize address structure.

Modbus has three major variations, which are Modbus Serial RTU, Modbus Serial ASCII and Modbus TCP/IP.

Modbus ASCII — This was the first version that was created. It is a serial communication and makes use of ASCII characters for protocol communication. The ASCII format uses a longitudinal redundancy check checksum.

Modbus RTU — This is also a serial communication which today is more predominant than the Modbus ASCII version. It makes use of a compact, binary representation of the data for protocol communication.

Modbus TCP/IP or Modbus TCP — This is a Modbus variant used for communications over TCP/IP networks, connecting over port 502.

The Serial versions were the original implementations. They are usually a master/slave communication protocol where the master device (which could be PLCs or HMI ) initiates the transmission of data across the network. The physical layer (which simply means the electrical wiring) is not standardized. Modbus can operate on RS232, RS422, RS485 and even over ethernet. This might necessitate converters when connecting devices from different manufacturers.

The number of network nodes on a Modbus network depends on the implementation. RS232 permits communication between a single pair of devices with a maximum distance of 50 feet. RS 422 permits 10 devices over a combined distance of about 500 feet. RS 485 permits up to 247 devices over a maximum distance of 4000 feet. The ethernet implementation permits devices to be clients and servers simultaneously.

A Modbus command contains the Modbus address of the device it is intended for (1 to 247). Only the addressed device will respond and act on the command, even though other devices might receive it (an exception is specific broadcastable commands sent to node 0, which are acted on but not acknowledged)

In the video on the page you’ll find a demonstration of a typical application of Modbus protocol to control a Variable Frequency Drive(VFD). The advantages of this over the traditional voltage and current controls is explained in depth.

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