Modbus Write

Write data to server device register(s) over RS485 network

Since R2025a

• 

Libraries:
STM32 Microcontroller Blockset / STM32F7xx Based Boards
STM32 Microcontroller Blockset / STM32G4xx Based Boards
STM32 Microcontroller Blockset / STM32H7xx Based Boards

Description

The Modbus Write block writes data to the register(s) of the server device.

The Modbus Write block accepts data of type uint16 for single register write operations and [Nx1] vector data of type uint16 for multiple register write operations.

STM32™ Microcontroller Blockset supports the Modbus^® communication protocol over the RS485 network. The client and the server must be connected on the same RS485 network for successful Modbus communication. Each device on the RS485 network is referenced by a unique 8-bit address or identifier.

This table categories the server device registers referenced by the Modbus client device.

Register Type	Register Size	Allowed Client Operation on Register
Coil	1-bit	Read and Write
Discrete Input	1-bit	Read
Holding Register	16-bit	Read and Write
Input Register	16-bit	Read

The client can perform either a read or a write operation (depending on the register type) on register(s) of a server device.

Note

Ensure that the number of client write registers does not exceed the server read register count.

To enable the Modbus communication on a particular UART, navigate to the Configuration Parameters > Hardware Implementation > Target hardware resources > USART/UART properties of the model and perform the following:

1. Enable the Modbus on required USART/UART peripheral.

2. Select the MODBUS Mode parameter as Server or Client. If you set the MODBUS Mode to Server, configure the registers accordingly.

3. Select the register type to configure and enter the register address in the Start address field. For more information, see UART/USART.

Examples

MODBUS RS485 Communication Between Client and Server Devices Using STMicroelectronics Discovery Board

Use the STM32™ Microcontroller Blockset to implement a MODBUS® RS485 asynchronous, serial communication between MODBUS client and server devices. The example also shows the four modes of operation: Client Read, Client Write, Server Read, and Server Write.

• Open Model

Ports

Input

data — Input data

vector

The input to the block is the data to be written to the server register(s) by the client as a N-by-1 vector, where N represents the number of registers on the server device.

• If you select the Coil or Discrete Input write operations, the client writes data to the server registers as a 1-bit write operation having the Boolean data type.

• If you select the Holding Register or Input Register write operations, the client writes data to the server registers as a 16-bit write operation having the uint16 data type.

Data Types: Boolean | uint16

Parameters

Block type — Modbus block type

Client (default) | Server

Select the Modbus write block type.

Programmatic Use

Block Parameter: BlockTyp

Server ID — Unique 8-bit Modbus server identifier

0 (default) | scalar in the range [1, 255]

Enter the unique 8-bit Modbus server ID or identifier to identify the server to which the client writes the data.

Programmatic Use

Block Parameter: ServerId

Register type — Type of write operation

Coil (default) | Discrete input | Holding register | Input register

Select the type of write operation you want to perform on the server register(s). Specify any one of the following:

• Coil: Write data to the coil register starting at the address specified in the Coil Address parameter.

• Discrete input: Write data to the discrete input register starting at the address specified in the Discrete Input Address parameter. Writing data to the discrete input register address is not supported in client mode

• Holding register: Write data to the holding register starting at the address specified in the Holding Register Address parameter.

• Input register: Write data to the input register starting at the address specified in the Input Register Address parameter.

Programmatic Use

Block Parameter: RegType

Address — Start address of the write command

0 (default) | scalar

Specify the start address to execute the write operation. Use zero-based indexing; for example, to read from the 10th location, specify the value as 9.

Programmatic Use

Block Parameter: WriteAddress

Number of registers — Number of registers for the client to write

1 (default) | scalar

Specify the number of registers the client writes data to. For example, for the client to write data from the coil registers with location ranging from 10 to 14, enter 5.

Programmatic Use

Block Parameter: Number

UART/USART module — USART or UART module for MODBUS communication

USART1 (default) | USART2 | UART4

Configure the USART or UART module for MODBUS communication.

To enable read of a particular register, navigate to the Configuration Parameters > Hardware Implementation > Target hardware resources > USART/UART properties.

Programmatic Use

Block Parameter: UARTModule

Timeout — Allowed time to complete operations

0.1 (default) | scalar

Amount of time in seconds that the model waits for data during each simulation time step.

Programmatic Use

Block Parameter: Timeout

Sample Time — Time interval at which client writes data

1 (default) | scalar

Enter the time interval (in seconds) at which the client writes data from the server device register(s).

Programmatic Use

Block Parameter: SampleTime

Version History

Introduced in R2025a