What is Modbus?

In the real world of the device to device process automation communication, a dialogue or conversation between devices occurs systematically, in many cases, over different types of communication networks and in different languages.

Modbus as an “Open Protocol”

Depending on the process automation equipment manufacturer, a very specific or proprietary language is used or it may be a language that is commonly open to the industry. It is these open protocols many manufacturers adapt to easily integrate their products in a market.

An “Open protocol” means the specifications are published and may be used by anyone freely or by license.

Open protocols are usually backed by a combination of corporations, user groups, professional societies, and governments. This provides users with a much wider choice of devices or systems that can be utilized to meet specific applications.

Advantages of open protocols include support by multiple manufacturers, software vendors, and install or service organizations, active community groups for support, the ability to stay current and add capabilities in the future.

One of the most common Automation Communication Protocols of connecting industrial electronic devices used today is Modbus.

In this article, we will discuss what Modbus communication protocol is and the types of Modbus communication used in the industry.

What is Modbus?

The Modbus communication protocol is the oldest and by far the most popular automation protocol in the field of process automation and SCADA (Supervisory Control and Data Acquisition).

Knowing how to create Modbus based networks is essential for any electrical technician and engineer working in these occupation fields.

Being able to integrate devices from different manufacturers is a skill that is in demand and will ultimately make you more valuable and marketable in the industry.

Modbus is a communications protocol published by Modicon in 1979 for use with its Programmable Logic Controllers (PLCs). Modicon is now owned by Schneider Electric.

Modbus provides a common language for devices and equipment to communicate with one and another.

For example, Modbus enables devices on a system that measures temperature and humidity connected on the same network to communicate the results to a supervisory computer or PLC.

The development and update of Modbus protocols have been managed by the Modbus Organization. The Modbus Organization is an association of users and suppliers of Modbus-compliant devices.

Types of Modbus Communication Protocol

Several versions of the Modbus protocol exist for the serial port and Ethernet and the most common are:

– Modbus RTU

– Modbus ASCII

– Modbus TCP

– Modbus Plus

Modicon published the Modbus communication interface for a multidrop network based on a Master-Slave architecture.

Communication between the Modbus nodes is achieved with send request and read response type messages.

Modbus Protocol and its Physical Media

Modbus is an open standard that describes the messaging communication dialog.

Modbus communicates over several types of physical media such as:

– Serial RS-232

– Serial RS-485

– Serial RS-422

– Ethernet

The original Modbus interface ran on RS-232 serial communication, but most of the later Modbus implementations use RS-485 because it allowed:

– Longer distances.

– Higher speeds.

– The possibility of multiple devices on a single multi-drop network.

Master-Slave Modbus communication over serial RS-485 physical media using two-wire transmit and receive connections.

On simple interfaces like RS232 or RS485, the Modbus messages are sent in plain form over the network and the network will be dedicated to only Modbus communication.

However, if your network requires multiple heterogeneous devices using a more versatile network system like TCP/IP over Ethernet, the Modbus messages are embedded in Ethernet packets with the format prescribed for this physical interface.

So in this case, Modbus and other types of mixed protocols can co-exist at the same physical interface at the same time.

Introduction to Modbus Message Structure

The main Modbus message structure is Peer-to-Peer. Modbus is able to function on both Point-to-Point and Multidrop networks.

Modbus devices communicate using a Master-Slave (Client-Server for Ethernet) technique in which only one device (the Master/Server) can initiate transactions (called queries).

The other devices (Slaves/Clients) respond by supplying the requested data to the master, or by taking the action requested in the query.

A slave is any peripheral device such as an I/O transducer, valve, network drive, or other measuring types of devices which processes information and sends its response message to the master using Modbus.

Masters can address individual slaves or initiate a broadcast message to all slaves. Slaves return a response to all message queries addressed to them individually, but do not respond to broadcast messages.

Slaves do not initiate messages on their own and only respond to message queries transmitted from the master.

The master’s query will consist of:

– Slave address (broadcast address).

– Function code with a read or write data command to the slave.

– The write command “Data” if a write command was initiated by the master.

– Error checking field.

The error checking is a value the master or slave creates at the beginning of the transmission or response and then checked when the message is received to verify the contents are correct.

A slave’s response consists of:

– Fields confirming it received the request.

– The data to be returned.

– Error checking data.

If no error occurs, the slave’s response contains the data as requested.

If an error occurs in the message query received by the slave, or if the slave is unable to perform the action requested, the slave will return an exception message as its response.

The error check field of the slave’s message frame allows the master to confirm that the contents of the message are valid.

In this article, you learned about what Modbus communication protocol is and the types of Modbus communication used in the industry.

In the next article, we will discuss the characteristics of how Modbus communication protocol works between devices.

What’s the biggest “aha” you’re taking away from this technical conversation? How can you put that insight into action now? Tell us the comments below.

Thanks again for reading. Leave your questions and comments and we’ll chat with you soon!

Happy learning,

The RealPars Team

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