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Modbus Communication Protocol: A Complete Guide for Industrial Automation

Mod Bus: A Data Communication Protocol for Industrial Devices

Mod bus is a data communication protocol that was developed by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Mod bus has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices.

mod bus

In this article, we will explain what mod bus is, how it works, what are its advantages and limitations, and what are the different types of mod bus used in the industry. We will also answer some frequently asked questions about mod bus.

What is Mod Bus?

Mod bus is a communication protocol that provides a common language for devices and equipment to communicate with one another. For example, mod bus 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.

Mod bus is based on a master-slave (or client-server) architecture, where one device (the master or client) initiates a request and one or more devices (the slaves or servers) respond to the request. Mod bus supports communication to and from multiple devices connected to the same cable or Ethernet network.

Mod bus is popular in industrial environments because it is openly published and royalty-free. It was developed for industrial applications, is relatively easy to deploy and maintain compared to other standards, and places few restrictions on the format of the data to be transmitted.

How does Mod Bus work?

Mod bus uses character serial communication lines, Ethernet, or the Internet protocol suite as a transport layer. Mod bus messages are composed of binary or ASCII characters that follow a specific format. Each message contains information such as the device address, the function code, the data, and the error check.

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The device address identifies the slave device that should respond to the request. The function code specifies what kind of action or data is requested or provided. The data contains the actual values or parameters that are exchanged between the devices. The error check ensures the integrity of the message during transmission.

The master device sends a request message to one or more slave devices and waits for their response. The slave devices process the request and send back a response message to the master device. The response message may contain an acknowledgment, an error code, or the requested data. The master device can then process the response and send another request if needed.

What are the advantages of Mod Bus?

Some of the advantages of mod bus are:

  • It is an open and widely used protocol that can be implemented by anyone freely or by license.

  • It is supported by multiple manufacturers, software vendors, and service organizations, which provides users with a wider choice of devices or systems that can be integrated.

  • It is simple and flexible, which makes it easy to learn and use. It can handle various types of data, such as discrete inputs/outputs, analog inputs/outputs, registers, coils, etc.

  • It is reliable and robust, which makes it suitable for harsh industrial environments. It has built-in error detection mechanisms that ensure data accuracy and integrity.

What are the limitations of Mod Bus?

Some of the limitations of mod bus are:

  • It has a limited number of data types that are understood by PLCs at the time of its development. It does not support large binary objects or complex data structures.

  • It does not provide a standard way for a device to find the description of a data object, such as its unit, range, or meaning. The user has to know beforehand what each data object represents and how to interpret it.

  • It does not support event-driven communication, where a device can send data when it changes or when triggered by an event. The master device has to poll each slave device periodically and look for changes in the data, which may cause network congestion or data loss.

  • It does not provide any security features, such as encryption, authentication, or authorization. The data transmitted over mod bus is vulnerable to eavesdropping, tampering, or unauthorized access.

What are the different types of Mod Bus?

There are mainly two types of mod bus: mod bus serial and mod bus TCP/IP.

Mod Bus Serial

Mod bus serial is the original version of mod bus that uses serial communication lines, such as RS-232, RS-485, or RS-422, to connect devices. Mod bus serial supports two transmission modes: binary (RTU) and ASCII. In binary mode, each character is sent as a byte (8 bits), which makes the transmission faster and more compact. In ASCII mode, each character is sent as two hexadecimal digits (4 bits each), which makes the transmission more readable and compatible with other devices.

Mod bus serial can use different physical network topologies, such as point-to-point, multidrop, or multipoint. The most common topology is multidrop, where one master device and up to 247 slave devices are connected to a single pair of wires. The master device can communicate with any slave device by using its unique address.

Mod Bus TCP/IP

Mod bus TCP/IP is a newer version of mod bus that uses Ethernet or the Internet protocol suite as a transport layer. Mod bus TCP/IP encapsulates the mod bus message inside a TCP/IP packet, which enables mod bus communication over any TCP/IP network, such as LAN, WAN, or the Internet. Mod bus TCP/IP does not require any additional hardware or software to implement, as it uses the existing network infrastructure and devices.

Mod bus TCP/IP can support multiple masters and multiple slaves on the same network. Each device has a unique IP address that identifies it on the network. The master device can communicate with any slave device by using its IP address and port number. Mod bus TCP/IP also supports broadcasting, where a master device can send a request to all slave devices at once.

A Comparison of Mod Bus Serial and Mod Bus TCP/IP

The following table summarizes some of the main differences between mod bus serial and mod bus TCP/IP:

Mod Bus Serial

Mod Bus TCP/IP

Uses serial communication lines

Uses Ethernet or the Internet protocol suite

Supports binary (RTU) or ASCII transmission modes

Supports only binary (RTU) transmission mode

Uses device addresses (0-247)

Uses IP addresses and port numbers

Supports one master and up to 247 slaves per network

Supports multiple masters and multiple slaves per network

Does not support broadcasting

Supports broadcasting

Requires additional hardware or software converters to connect to other networks

Does not require any additional hardware or software converters to connect to other networks

Has lower data throughput and higher latency

Has higher data throughput and lower latency

Has lower network complexity and cost

Has higher network complexity and cost


In conclusion, mod bus is a data communication protocol that enables industrial devices and equipment to communicate with one another. Mod bus has two main types: mod bus serial and mod bus TCP/IP, which have different characteristics and advantages. Mod bus is widely used in industrial applications because it is simple, flexible, reliable, and open.

Frequently Asked Questions about Mod Bus

What are some examples of devices that use mod bus?

Some examples of devices that use mod bus are PLCs, sensors, actuators, meters, controllers, inverters, drives, valves, switches, relays, etc.

What are some applications of mod bus?

Some applications of mod bus are automation systems, building management systems, smart grid systems, renewable energy systems, water treatment systems, oil and gas systems, etc.

How can I test or monitor mod bus communication?

You can use various software tools or hardware devices that can act as a mod bus master, slave, sniffer, simulator, or analyzer. Some examples of these too


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