Learn the basics of Modbus, the types of messages, the addressing scheme, and the tools and software to program Modbus devices in a plant automation system.

No son tipos de mensajes. Son tramas de protocolos diferentes. Un protocolo es el modbus ASCII, otro el Modbus RTU y otro diferente es el Modbus TCP. Leyendo esto puede parecer que se pueden mezclar mensajes ASCII y RTU en una misma comunicación cuando no es así. El formato de las tramas es siempre el mismo pero la forma en la que se comunican los equipos, no. Tienes que elegir uno de los tres protocolos para configurar las comunicaciones de tu equipo.

How do you program Modbus devices in a plant automation system?

Modbus is a widely used industrial network protocol that allows communication between various devices in a plant automation system. It is based on a master-slave or client-server model, where one device initiates a request and another device responds with data or an action. To program Modbus devices, you need to understand the basics of Modbus, the types of Modbus messages, the addressing scheme, and the tools and software available.

1 Basics of Modbus

Modbus is a simple and robust protocol that can be implemented on different physical layers, such as serial (RS-232, RS-485) or Ethernet (TCP/IP). Modbus uses a common data format, which consists of a function code and a data field. The function code specifies the type of operation to be performed, such as reading or writing registers, coils, or discrete inputs. The data field contains the address and the value of the data to be exchanged.

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Andrew Arsenault

Industrial controls engineering
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Modbus devices work off of a Master/slave configuration. The Modbus master is the device that initiates communication with Modbus slave devices. It's parameters need to be configured as such: The communication protocol (Modbus RTU, Modbus TCP, or Modbus ASCII) The baud rate (for Modbus RTU) The IP address and port number (for Modbus TCP) The slave addresses of the devices to communicate with

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2 Types of Modbus messages

There are two main types of Modbus messages: Modbus RTU and Modbus ASCII for serial communication, and Modbus TCP for Ethernet communication. Modbus RTU and Modbus ASCII differ in the way they encode the data and the error checking method. Modbus RTU uses binary encoding and a cyclic redundancy check (CRC), while Modbus ASCII uses hexadecimal encoding and a longitudinal redundancy check (LRC). Modbus TCP uses the same data format as Modbus RTU, but adds a header and a trailer to the message to conform to the TCP/IP protocol.

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Leopoldo Ferrer

ICS/Scada Blue Dragon Senior Specialist
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No son tipos de mensajes. Son tramas de protocolos diferentes. Un protocolo es el modbus ASCII, otro el Modbus RTU y otro diferente es el Modbus TCP. Leyendo esto puede parecer que se pueden mezclar mensajes ASCII y RTU en una misma comunicación cuando no es así. El formato de las tramas es siempre el mismo pero la forma en la que se comunican los equipos, no. Tienes que elegir uno de los tres protocolos para configurar las comunicaciones de tu equipo.

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Andrew Arsenault

Industrial controls engineering
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Begin by exploring simple examples to familiarize yourself with Modbus messaging: Read Coil Status (FC01): Read the status of a single coil (ON/OFF) on a slave device. Read Input Status (FC02): Read the status of multiple inputs on a slave device. Read Holding Registers (FC03): Read the contents of multiple holding registers (data storage) on a slave device. Write Single Coil (FC05): Change the state of a single coil on a slave device. Write Single Register (FC06): Update the value of a single holding register on a slave device.

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3 Addressing scheme

Modbus uses a 16-bit address space to identify the data points in a device. The address range is from 0 to 65535, but some addresses are reserved for specific functions. For example, addresses from 1 to 9999 are used for coils and discrete outputs, addresses from 10001 to 19999 are used for discrete inputs, addresses from 30001 to 39999 are used for input registers, and addresses from 40001 to 49999 are used for holding registers. The address of a data point is also called the offset, which is the distance from the beginning of the address range.

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Andrew Arsenault

Industrial controls engineering
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Modbus devices store data in different types of memory blocks, each with its own address range: Coils (FC01, FC05): Coils represent discrete states, typically representing binary inputs or outputs. Their addresses range from 00001 to 65535 (1 to 65536). Discrete Inputs (FC02): represent the current status of external sensors or switches. Their addresses range from 10001 to 65535 (10001 to 65536). (FC03, FC06): Holding registers store data that can be read or written by the master. They are typically used for holding configuration parameters or process variables. Their addresses range from 40001 to 65535 (40001 to 65536). (FC04): Input registers are read-only registers that contain analog or process data directly from the slave device.

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4 Tools and software

To program Modbus devices, you need a Modbus master device, such as a PLC, a PC, or a controller, that can send and receive Modbus messages. You also need a Modbus slave device, such as a sensor, an actuator, or a meter, that can respond to Modbus requests. Additionally, you need a tool or software that can configure the parameters of the Modbus devices, such as the baud rate, the parity, the stop bits, the unit ID, and the data format. Some examples of Modbus tools and software are Modbus Tester, Modscan, Modbus Poll, and Modbus Slave.

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Luis Rodriguez L.

Automation Engineer | ICS Specialist | PLC | Drives | SCADA
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Modbus Poll, Slave y Modscan son softwares muy importantes al momento de diagnosticar y/o comisionar equipos en este protocolo, ya que permite descartar fallos a nivel de la capa 7 de OSI, muy recomendado.

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5 Programming steps

To program Modbus devices, you must first connect them to the network using the appropriate physical layer and wiring. Then, set up the communication parameters of the Modbus devices with a tool or software. Assign a unique unit ID to each Modbus slave device on the network and define the data points and addresses of the Modbus devices according to the data type and function code. After that, you can write the Modbus requests and responses with a tool or software or use a programming language such as C, Python, or Java. Finally, test and debug the Modbus communication with a tool or software or use a Modbus simulator or analyzer.

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6 Programming tips

Programming Modbus devices effectively requires following certain tips, such as using a consistent naming convention for the data points and the addresses of the Modbus devices, documenting the Modbus configuration and data map for future reference and maintenance, relying on a library or framework that simplifies coding and error handling of Modbus messages, utilizing a gateway or converter for interoperability between different Modbus variants or other protocols, and using a monitor or logger to record and analyze Modbus traffic on the network.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you program Modbus devices in a plant automation system?

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1 Basics of Modbus

2 Types of Modbus messages

3 Addressing scheme

4 Tools and software

5 Programming steps

6 Programming tips

7 Here’s what else to consider

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