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Murad Yakushev
Murad Yakushev

Modbus: The Everyman's Guide to the Best Hardware, Software, and Tools for Modbus Implementation and Testing



Modbus: The Everyman's Guide To




Introduction




If you are interested in industrial automation, smart home, or IoT projects, you have probably heard of Modbus. But what is Modbus exactly? And why is it so widely used in different applications and devices? In this article, we will answer these questions and more. We will explain what Modbus is, how it works, and how you can use it in your own projects. By the end of this article, you will have a solid understanding of Modbus and its benefits.




Modbus: The Everyman's Guide To


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What is Modbus?




Modbus is a communication protocol that allows devices to exchange data over serial or Ethernet networks. It was developed in 1979 by Modicon, a company that specialized in programmable logic controllers (PLCs). Modbus was designed to be simple, robust, and easy to implement on various devices and platforms. It is an open and royalty-free protocol that has become a de facto standard in industrial automation and other fields.


Why is Modbus so popular?




Modbus has several advantages that make it popular among users and developers. Some of these advantages are:



  • It is compatible with a wide range of devices, such as PLCs, sensors, actuators, meters, controllers, etc.



  • It supports multiple communication modes, such as ASCII, RTU, TCP/IP, UDP/IP, etc.



  • It supports multiple data types, such as coils, registers, discrete inputs, input registers, etc.



  • It has a simple and flexible addressing scheme that allows up to 247 devices on a serial network and up to 65535 devices on an Ethernet network.



  • It has a rich set of functions that allow reading, writing, masking, diagnostic, etc. operations on data.



  • It has a built-in error handling mechanism that ensures data integrity and reliability.



  • It is easy to implement and configure on various hardware and software platforms.



  • It is widely supported by many vendors and communities that offer hardware, software, tools, libraries, documentation, etc.



How does Modbus work?




Modbus works on a master-slave or client-server model. This means that there is one device that initiates the communication (the master or client) and one or more devices that respond to the communication (the slaves or servers). The master or client sends a request message to the slave or server with a specific function code and data address. The slave or server then performs the requested function and sends back a response message with the result or an error code. The communication can be either one-to-one or one-to-many depending on the network topology and configuration.


Modbus communication modes




Modbus supports two main communication modes: serial and Ethernet. Serial mode uses physical wires (such as RS-232, RS-485, etc.) to connect devices on a network. Ethernet mode uses TCP/IP or UDP/IP protocols to connect devices on a network. Each mode has its own advantages and disadvantages depending on the application requirements.


Serial mode has two sub-modes: ASCII and RTU. ASCII mode uses human-readable characters to encode data in hexadecimal format. RTU mode uses binary format to encode data in compact form. ASCII mode is easier to debug and troubleshoot, but RTU mode is faster and more efficient.


Ethernet mode has two sub-modes: TCP/IP and UDP/IP. TCP/IP mode uses a connection-oriented protocol that ensures reliable and ordered delivery of data. UDP/IP mode uses a connectionless protocol that does not guarantee reliability or order of data, but is faster and more lightweight.


Modbus data types




Modbus supports four main data types: coils, registers, discrete inputs, and input registers. Each data type has a specific function and format. Coils are single-bit values that can be read or written by the master or client. Registers are 16-bit values that can be read or written by the master or client. Discrete inputs are single-bit values that can only be read by the master or client. Input registers are 16-bit values that can only be read by the master or client.


Coils and registers can be used to store and manipulate various kinds of data, such as boolean, integer, floating-point, string, etc. However, Modbus does not define the data format or interpretation of these data types. It is up to the user or developer to define how the data is encoded and decoded on each device.


Modbus addressing




Modbus uses a simple and flexible addressing scheme that allows up to 247 devices on a serial network and up to 65535 devices on an Ethernet network. Each device has a unique address that identifies it on the network. The address can be either a number (0-247 for serial mode and 0-65535 for Ethernet mode) or a name (such as COM1, COM2, etc. for serial mode and IP address, hostname, etc. for Ethernet mode).


Each device also has a data address that identifies the location of the data within the device. The data address can be either a number (0-9999 for coils and discrete inputs and 0-65535 for registers and input registers) or a name (such as %Q0.0, %Q0.1, etc. for coils and discrete inputs and %MW0, %MW1, etc. for registers and input registers).


The master or client can access the data on the slave or server by specifying the device address and the data address in the request message. For example, to read the coil at address %Q0.0 on the device at address 1, the master or client would send a request message with function code 1 (read coil) and data address 0.


Modbus functions




Modbus has a rich set of functions that allow reading, writing, masking, diagnostic, etc. operations on data. Each function has a specific code that identifies it in the request and response messages. Some of the most common functions are:



  • Function code 1: Read coil - This function allows the master or client to read one or more coils from the slave or server.



  • Function code 2: Read discrete input - This function allows the master or client to read one or more discrete inputs from the slave or server.



  • Function code 3: Read holding register - This function allows the master or client to read one or more holding registers from the slave or server.



  • Function code 4: Read input register - This function allows the master or client to read one or more input registers from the slave or server.



  • Function code 5: Write single coil - This function allows the master or client to write a single coil to the slave or server.



  • Function code 6: Write single register - This function allows the master or client to write a single register to the slave or server.



  • Function code 15: Write multiple coils - This function allows the master or client to write multiple coils to the slave or server.



  • Function code 16: Write multiple registers - This function allows the master or client to write multiple registers to the slave or server.



  • Function code 22: Mask write register - This function allows the master or client to modify bits in a register using a mask.



  • Function code 23: Read/write multiple registers - This function allows the master or client to read and write multiple registers in a single message.



There are many other functions that perform diagnostic, file transfer, report slave ID, etc. operations on data. You can find more information about these functions in the Modbus specification document.


Modbus error handling




Modbus has a built-in error handling mechanism that ensures data integrity and reliability. If an error occurs during communication, such as invalid function code, invalid data address, invalid data value, timeout, etc., the slave or server will send back an exception response message with an error code. The master or client can then handle the error accordingly.


Some of the common error codes are:



  • Error code 1: Illegal function - This error occurs when the master or client requests a function that is not supported by the slave or server.



  • Error code 2: Illegal data address - This error occurs when the master or client requests a data address that is not valid or accessible by the slave or server.



  • Error code 3: Illegal data value - This error occurs when the master or client requests a data value that is not valid or acceptable by the slave or server.



  • Error code 4: Slave device failure - This error occurs when the slave or server encounters an unrecoverable error that prevents it from performing the requested function.



  • Error code 5: Acknowledge - This error occurs when the slave or server accepts a long-duration request and needs more time to process it.



  • Error code 6: Slave device busy - This error occurs when the slave or server is busy processing another request and cannot accept a new one.



There are other error codes that indicate specific conditions or events on the slave or server. You can find more information about these error codes in the Modbus specification document.


How to use Modbus in your projects?




Now that you have learned what Modbus is and how it works, you might be wondering how to use it in your own projects. The good news is that Modbus is easy to implement and configure on various hardware and software platforms. All you need are some basic components and tools to get started.


Modbus hardware and software requirements




To use Modbus in your projects, you will need the following hardware and software components:



  • A Modbus master or client device - This can be any device that can initiate and control the communication, such as a PC, a PLC, a microcontroller, etc. You will need to install or program a Modbus master or client software on this device to send and receive Modbus messages.



  • A Modbus slave or server device - This can be any device that can respond to the communication, such as a sensor, an actuator, a meter, a controller, etc. You will need to install or program a Modbus slave or server software on this device to process and execute Modbus messages.



  • A Modbus network - This can be either a serial or an Ethernet network that connects the master or client and the slave or server devices. You will need to use appropriate cables, connectors, converters, switches, routers, etc. to establish and maintain the network.



Modbus configuration and testing tools




To use Modbus in your projects, you will also need some tools to configure and test the communication. Some of these tools are:



  • A Modbus scanner - This is a software tool that can scan and detect Modbus devices on a network. It can also read and write data from and to these devices using various Modbus functions. A Modbus scanner can help you troubleshoot and debug your Modbus communication.



  • A Modbus simulator - This is a software tool that can simulate a Modbus master or client or a Modbus slave or server on a PC. It can also generate and analyze Modbus messages using various Modbus functions. A Modbus simulator can help you test and verify your Modbus communication without using real devices.



  • A Modbus analyzer - This is a hardware or software tool that can capture and decode Modbus messages on a network. It can also display and analyze various parameters and statistics of the communication. A Modbus analyzer can help you monitor and optimize your Modbus communication.



Modbus application examples




Modbus can be used in many different applications and domains. Some of the common examples are:



  • Industrial automation - Modbus can be used to connect various industrial devices, such as PLCs, sensors, actuators, meters, controllers, etc., to monitor and control processes and systems.



  • Smart home - Modbus can be used to connect various smart devices, such as thermostats, lights, cameras, locks, etc., to create and manage a smart home environment.



  • IoT - Modbus can be used to connect various IoT devices, such as sensors, actuators, gateways, cloud services, etc., to collect and transmit data and perform actions.



Conclusion




Modbus is a communication protocol that allows devices to exchange data over serial or Ethernet networks. It is simple, robust, and easy to implement on various devices and platforms. It is an open and royalty-free protocol that has become a de facto standard in industrial automation and other fields. It supports multiple communication modes, data types, addressing schemes, functions, and error handling mechanisms. It is easy to use and configure on various hardware and software platforms. It can be used in many different applications and domains.


FAQs




Here are some frequently asked questions about Modbus:



  • What is the difference between Modbus RTU and Modbus TCP?



  • How many devices can be connected on a Modbus network?



  • How fast can Modbus communicate?



  • How secure is Modbus?



  • Where can I find more information about Modbus?



Here are the answers:



  • Modbus RTU is a serial communication mode that uses binary format to encode data in compact form. Modbus TCP is an Ethernet communication mode that uses TCP/IP protocol to ensure reliable and ordered delivery of data.



  • The number of devices that can be connected on a Modbus network depends on the communication mode and the addressing scheme. For serial mode, up to 247 devices can be connected on a network using numerical addresses. For Ethernet mode, up to 65535 devices can be connected on a network using numerical or name addresses.



  • The speed of Modbus communication depends on the communication mode and the baud rate or the network bandwidth. For serial mode, the baud rate can range from 1200 bps to 115200 bps. For Ethernet mode, the network bandwidth can range from 10 Mbps to 1000 Mbps.



  • Modbus is not a secure protocol by itself. It does not provide any encryption or authentication mechanisms to protect the data from unauthorized access or modification. However, Modbus can be combined with other security protocols or methods, such as SSL/TLS, VPN, firewall, etc., to enhance the security of the communication.



  • You can find more information about Modbus on the official website of Modbus Organization (www.modbus.org), which is a non-profit association that maintains and promotes the protocol. You can also find various resources, such as specification documents, application notes, software libraries, hardware products, tools, forums, etc., on this website.



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