Understanding the MODBUS Protocol for Process Control & Automation

Industries across the world use MODBUS protocol for various advantages it offers. According to the market data, it is most popular industrial protocol that is simple, cost-effective, and globally accepted. It is pretty easy to set up MODBUS connection in a new as well as an existing plant.

Even after being a decades old mechanism, it is used in many engineering products of present generation. Thanks to the facilitations offered by it! One of the greatest perks of using MODBUS is that it virtually runs on a variety of communication mediums, such as twisted wires, fiber optics, Ethernet, wireless, cell phone and microwave.

To get more technical, it is a master-slave system, wherein the master mechanism controls or is responsible for the actions of slave client(s). While the master client is a Programmable Logical Controller (PLC), the slaves are characteristically field devices, which are connected to each other through a network.

Master and Slave Devices

In a MODBUS device, the master client sends messages, and slaves, who do not really work on their own, are designed to accept or receive the communications or requests sent by the former. To generalize it, slave clients do only what they have been asked to perform by their master. Master client can send message to any slave present in the environment, and the slave, whom the message is sent, will only take the request, although the request will be visible to other slaves as well.

MODBUS Versions

After its inception in the year 1979, there have been major developments. Presently, there are three versions -

·         MODBUS ASCII

·         MODBUS RTU

·         And, MODBUS/TCP

Receiving messages in hexadecimal codes and 4-bit ASCII characters, MODBUS ASCII is the slowest MODBUS gateway. It makes the best fit to be used for telephone modems and RF frequencies. On the other hand, MODBUS RTU works with binary codes, and with a speed of up to 19200 bauds, it is a faster protocol ideal for many industrial applications. And, the MODBUS/TCP can be termed as a protocol with a difference. It employs IP Addresses to connect with slave clients, not the address or the location of the device.

Process Control & Automation

There is a memory map in every MODBUS device, which stores several sets of important data. Retrieving and accessing data for a master and user is straightforward. The operators can easily access the stored data, by going through the specification and instruction manual.

Connecting the digital and analog I/O to a distributed I/O, having the capability of MODBUS communication, is the easiest way you can monitor the performance of a field device (slave) through a PLC or industrial computer.

Sometimes, there are used repeaters and converters – such as MODBUS to BACnet gateway, BACnet to MODBUS gateway, CANbus to MODBUS gateway, etc. – to establish an easy to use industrial automation. It, once again, displays the versatility and simplicity of the MODBUS protocol.