What is Modbus and How does it Work?

Summary

The video script introduces the Modbus communication protocol, a widely used standard in process automation and SCADA systems. Modbus, developed by Modicon (now part of Schneider Electric) in 1979, facilitates communication between industrial electronic devices. It operates on various physical media, including RS-232, RS-485, and Ethernet, and supports both point-to-point and multidrop networks. The protocol uses a master/slave architecture where the master initiates transactions and slaves respond with requested data. Modbus messages are structured with error checking to ensure data integrity. The video also highlights the importance of understanding Modbus for professionals in the field, emphasizing its role in integrating devices from different manufacturers and its demand in the industry.

Takeaways

• 📈 **Open Protocols**: Open protocols like Modbus allow for device interoperability, enabling manufacturers to integrate their products easily into the market, providing users with a wide range of device choices.
• 🤝 **Community Support**: Open protocols are often backed by a diverse group including corporations, user groups, professional societies, and governments, offering robust community support.
• 🚀 **Modbus Protocol**: Modbus is a widely used communication protocol in process automation and SCADA systems, essential for electrical technicians and engineers to understand and implement.
• 🔌 **Modbus Versions**: Several versions of Modbus exist, including RTU, ASCII, TCP, and Plus, each designed for different types of communication media and network architectures.
• 💬 **Communication Architecture**: Modbus operates on a master/slave architecture, with the master device initiating transactions and the slave devices responding with the requested data.
• 🌐 **Physical Media**: Modbus can communicate over various physical media such as RS-232, RS-485, RS-422, and Ethernet, with the choice depending on the specific application and distance requirements.
• 🔄 **RS-485 Advantages**: RS-485 is preferred for Modbus implementations due to its support for longer distances, higher speeds, and the ability to connect multiple devices on a single network.
• 📨 **Modbus Messaging**: Modbus messages are sent in plain text over dedicated networks or embedded within Ethernet packets when co-existing with other protocols on TCP/IP networks.
• 📡 **Point-to-Point and Multidrop**: Modbus supports both point-to-point and multidrop networks, allowing for flexible communication setups.
• 🔑 **Master/Slave Communication**: In Modbus, only the master can initiate transactions, while slaves respond to queries and do not initiate communication unless addressed directly.
• 🛡️ **Error Checking**: Modbus includes error checking mechanisms to ensure the integrity of the data being transmitted between devices, with the master verifying the accuracy of the received information.

Q & A

What is the significance of using open protocols in device to device communication?

Open protocols allow for greater interoperability between devices from different manufacturers. They are published specifications that can be used freely or by license, enabling a wide range of devices or systems to be utilized for specific applications. This provides users with more choices and the ability to stay current and add capabilities in the future.

What are the advantages of using open protocols in industrial automation?

The advantages include support from multiple manufacturers, software vendors, and install/service organizations, active community groups for support, and the ability to integrate devices from different manufacturers, making professionals more valuable and marketable in the industry.

What is the Modbus communication protocol?

Modbus is a communication protocol published by Modicon in 1979 for use with its programmable logic controllers (PLCs). It provides a common language for devices and equipment to communicate with one another and is used extensively in process automation and SCADA systems.

What types of Modbus communication are commonly used in the industry?

The most common types of Modbus communication used in the industry are Modbus RTU, Modbus ASCII, Modbus TCP, and Modbus Plus. These versions cater to different needs and physical media such as serial ports and Ethernet.

How does Modbus enable communication between devices?

Modbus enables devices to communicate by allowing them to send and receive messages. For example, devices measuring temperature and humidity can communicate their results to a supervisory computer or PLC using Modbus.

What is the role of the Modbus Organization?

The Modbus Organization is an association of users and suppliers of Modbus-compliant devices. It manages the development and updates of Modbus protocols.

What is the architecture of Modbus communication interface?

The Modbus communication interface is based on a master/slave architecture. Communication between Modbus nodes is achieved with send request and read response type messages.

How does Modbus communication work over different physical media?

Modbus communicates over several types of physical media such as serial RS-232, RS-485, RS-422, and over Ethernet. The physical media is selected at the time of purchasing the devices. Modbus messages are sent in plain form over simple interfaces like RS-485 and RS-232, while over Ethernet, they are embedded in Ethernet packets.

What is the main message structure of Modbus?

The main Modbus message structure is peer-to-peer. It can function on both point-to-point and multidrop networks, using a master/slave technique where only the master can initiate transactions.

How does a Modbus master communicate with slaves?

A Modbus master communicates with slaves by addressing individual slaves or initiating a broadcast message to all slaves. The master's query consists of a slave address, a function code with a read or write data command, and an error checking field.

What happens if a Modbus slave encounters an error or is unable to perform a requested action?

If a slave encounters an error or is unable to perform the requested action, it will return an exception message as its response. The error check field in the slave's message frame allows the master to confirm the validity of the message contents.

What is the next step for someone interested in learning more about Modbus and PLC programming?

For further learning on Modbus and PLC programming, one can visit realpars.com, which offers easy-to-understand formats to enhance career prospects in these fields.

Outlines

00:00

📡 Introduction to Device Communication and Modbus Protocol

This paragraph introduces the concept of device-to-device communication in the automation industry, emphasizing the use of both proprietary and open protocols. Open protocols, such as Modbus, are highlighted for their benefits, including widespread support and the ability to integrate various devices from different manufacturers. The Modbus protocol, in particular, is discussed as a critical skill for technicians and engineers, given its prevalence in process automation and SCADA systems. The paragraph also provides a brief history of Modbus, its development by Modicon (now part of Schneider Electric), and the different versions of the protocol, including RTU, ASCII, TCP, and Plus. The Modbus Organization's role in managing the protocol is also mentioned.

05:00

🔌 Modbus Communication: Structure and Function

The second paragraph delves into the technical aspects of Modbus communication, explaining how messages are sent and received over a network. It covers the plain form of Modbus messages over a dedicated network and the embedding of Modbus messages in Ethernet packets for networks with multiple heterogeneous devices. The paragraph outlines the peer-to-peer structure of Modbus messages and their operation on both point-to-point and multidrop networks. The master/slave communication technique is described, with the master initiating transactions and slaves responding with requested data or actions. The message structure, including slave addressing, function codes, data commands, and error checking, is detailed. The response mechanism of slaves and the error handling process are also explained. The paragraph concludes with a teaser for the next video, which will further explore the characteristics of Modbus communication, and a call to action for learning PLC programming.

Keywords

💡Modbus

Modbus is a communication protocol designed for use with programmable logic controllers (PLCs) and other industrial devices. It was developed by Modicon in 1979 and is used to allow electronic devices to communicate over various types of networks. In the context of the video, Modbus is described as essential for any technician or engineer in process automation and SCADA systems. The video discusses various forms of Modbus, including Modbus RTU, ASCII, TCP, and Plus, indicating its versatility and widespread adoption in industry.

💡Open protocol

An open protocol refers to a communication standard whose specifications are published and accessible either freely or via license. The video emphasizes that such protocols, including Modbus, allow for broad interoperability among devices from different manufacturers, which benefits users by providing a wider choice of compatible equipment. Open protocols are typically supported by a mix of corporations, professional societies, and user groups.

💡SCADA

SCADA stands for Supervisory Control and Data Acquisition, a type of software and hardware system that allows industrial organizations to control processes locally or at remote locations, monitor, gather, and process real-time data. The video points out that knowing Modbus is crucial for working in SCADA systems, as it is one of the most commonly used protocols in these applications.

💡RS-485

RS-485 is a standard defining the electrical characteristics of drivers and receivers for use in serial communications systems. It is mentioned in the video as one of the physical media for Modbus communications, favored for its ability to support long distances, high speeds, and multiple devices on a single network, which is ideal for industrial environments.

💡Master/Slave

This refers to a communication model where one device (the master) controls one or more other devices (the slaves). In the context of the video, Modbus utilizes a master/slave architecture to manage communications in a network. Masters can send requests to slaves, which respond back but do not initiate communication, ensuring controlled and orderly data exchange.

💡Ethernet

Ethernet is a family of networking technologies commonly used in local area networks (LAN). The video discusses Modbus TCP, which is a version of the Modbus protocol that operates over Ethernet, allowing Modbus communication to integrate with more complex and capable network infrastructures, supporting a mix of different protocols.

💡Programmable Logic Controllers (PLCs)

PLCs are industrial digital computers which have been ruggedized and adapted to control manufacturing processes, such as assembly lines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis. Modbus was originally designed for PLCs by Modicon, now part of Schneider Electric, as highlighted in the video.

💡Broadcast message

In Modbus communication, a broadcast message is a special type of message sent by the master to all slaves simultaneously. Slaves do not respond to broadcast messages, which are typically used for sending commands that initiate identical or concurrent actions in multiple devices. This feature simplifies network management and reduces traffic.

💡Error checking

Error checking in Modbus communication involves verifying the integrity of message data using fields in the message frame. The video mentions that both the master and slave devices perform error checking to confirm that messages are not corrupted or altered during transmission, ensuring reliable and accurate communication between devices.

💡Peer-to-peer

Though primarily discussed in the context of Modbus' master/slave architecture, the video also mentions that Modbus supports a peer-to-peer message structure, which enables devices to function on a network either as points on a multidrop bus or in a more interconnected network. This flexibility is part of what has made Modbus a lasting standard in industrial automation.

Highlights

Device-to-device process automation communication occurs systematically over different networks and languages.

Open protocols allow manufacturers to integrate their products into the market more easily.

Open protocols are published specifications that can be used freely or by license.

Modbus is the most popular automation protocol in process automation and SCADA.

Modbus enables devices to communicate results to a supervisory computer or PLC.

Modbus was published by Modicon in 1979 for use with its PLCs.

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

The Modbus Organization manages the development and updates of Modbus protocols.

Modbus RTU, Modbus ASCII, Modbus TCP, and Modbus Plus are the most common versions.

Modbus operates on a master/slave architecture with send request and read response messages.

Modbus communicates over various physical media, including serial RS-232, RS-485, RS-422, and Ethernet.

Modbus messages are sent in plain form over simple interfaces like RS-485 and RS-232.

Modbus messages can be embedded in Ethernet packets for networks requiring multiple heterogeneous devices.

Modbus devices use a master/slave technique for communication, with only the master initiating transactions.

A slave device processes information and sends its response to the master using Modbus.

Masters can address individual slaves or initiate a broadcast message to all slaves.

Slaves do not initiate messages but respond to message queries from the master.

The master's query includes a slave address, function code, write command data, and an error checking field.

Slaves return an exception message if an error occurs or if they cannot perform the requested action.

The error check field allows the master to confirm the validity of the message contents.

Transcripts

In the real world of 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.

Depending on the process automation equipment manufacturer,

a very specific or proprietary language is used

or 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/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 lesson, we will discuss what Modbus communication protocol is

and the types of Modbus communication used in the industry.

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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 common language

for devices and equipment to communicate with one 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.

And 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.

Several versions of the Modbus protocol exist for the serial port

and Ethernet and the most common are Modbus RTU,

Modbus ASCII, Modbus TCP and 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 is an open standard that describes

the messaging communication dialog.

Modbus communicates over several types of physical media

such as serial RS-232, RS-485, RS-422 and over Ethernet.

The physical media will be selected at the time when you purchase the devices.

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 and the possibility of multiple devices on a single multi-drop network.

Master/Slave Modbus communication over serial RS-485 physical media

showing two-wire transmit and receive connections.

On simple interfaces like RS-485 and RS-232,

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.

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 can initiate transactions (called queries).

The other devices 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 a slave address (broadcast address),

a function code with a read or write data command to the slave,

along with the write command data

if a write command was initiated by the master,

and an 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, and an 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 video you learned about what Modbus communication protocol is

and the types of Modbus communication used in the industry.

In the next video, we will discuss the characteristics

of how Modbus communication protocol works between devices.

Stay tuned!

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