MQTT Client Publish

R2026b

Publish messages to MQTT topics

Since R2025a

• 

MQTT Client Publish block

To add a block to a model, double-click the canvas and start typing the block name. Then, select the block from the list.

Libraries:
Industrial Communication Toolbox / MQTT

Description

The MQTT Client Publish block publishes messages to one or more MQTT topics.

Each input port publishes messages to a corresponding MQTT topic specified in the Topics to publish parameter.

Note

You must associate the MQTT Client Publish block with an MQTT Client. You can create the client to connect to the MQTT broker using the MQTT pane in the Configuration Parameters dialog box of the model. You must add at least one MQTT Client Subscribe or MQTT Client Publish block in the model to enable this pane. For more information, see Configure MQTT Client in Simulink.

Examples

Industrial Process Monitoring Using MQTT

Monitor an industrial process using the MQTT Client blocks in Simulink®. You can connect to an MQTT broker using a Simulink model that acts as an MQTT client. You can publish messages to an MQTT topic using the MQTT Client Publish block and read messages from a topic using the MQTT Client Subscribe block.

• Since R2025a
• Open Live Script

Ports

Input

Topic_x — Messages to publish to MQTT topics

vector

Messages to publish to MQTT topics. The number of input ports corresponds to the number of topics you add in the Topics to publish parameter. The block publishes messages to topics when the input data changes. You can toggle this setting using the Publish only on input data change parameter.

Dependencies

To enable this port, specify one or more topics in the Topics to publish parameter.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | string

Parameters

Client — Select MQTT client

drop-down list

Select the MQTT client to associate with the block. The block updates the Broker address parameter once you select a client.

• To add the clients using the MQTT pane, click Configure clients. For more information, see Configure MQTT Client in Simulink.

• To update the client list and to show the latest MQTT clients, click Refresh.

Programmatic Use

To set the block parameter value programmatically, use the set_param (Simulink) function.

To get the block parameter value programmatically, use the get_param (Simulink) function.

Parameter:	ClientList
Values:	MQTT client name
Data Types:	character vector | string

Topics to publish — List of MQTT topics

table

Add topics to publish.

• To add a topic, click Add. This action adds a row in the Topics to publish table with QoS and Retain set to the default values of 0 and off, respectively. You must specify a topic name for the client to publish to in the Topic name column. Each time you click Add, the block adds a new row in the Topics to publish table and increments the serial number by 1.

  • Sl no: This field is noneditable and specifies the serial number of the topic. The serial number determines the order in which the block receives the signals at the input ports.

  • Topic name: This is an editable text field that specifies the name of the topic. This field also defines the names for the input ports.

  • QoS: This field specifies the quality of service (QoS) for message delivery. You can set this field to one of the values in the drop-down list:

    • 0: To deliver messages at most once and not more

    • 1: To deliver messages at least once and not less

    • 2: To deliver messages exactly once, not more or less

  • Retain: This field specifies whether the broker retains a message for new subscribers to the topic. Select this check box to retain the message.

• To delete topics, select the topics and click Delete.

Note

Topic names that begin with the $ character are not supported for publishing messages. These topics are reserved exclusively for server-side implementations and should not be used by clients.

Programmatic Use

To set the block parameter value programmatically, use the set_param (Simulink) function.

To get the block parameter value programmatically, use the get_param (Simulink) function.

Parameter:	TopicsTable
Values:	MQTT topic name and information
Data Types:	character vector | string

Publish only on input data change — Determines when to publish messages

on (default) | off

When you select this parameter, the block publishes messages to topics only when there is a change in input data.

Programmatic Use

To set the block parameter value programmatically, use the set_param (Simulink) function.

To get the block parameter value programmatically, use the get_param (Simulink) function.

Parameter:	PublishOnDataChange
Values:	on (default) | off
Data Types:	character vector | string

Block sample time — Sample time for publish operation

-1 (default) | nonnegative scalar

Define the sample time of the block in seconds. The block publishes messages to the MQTT topic synchronously at the specified sample time. For more information, see Specify Sample Time (Simulink).

Programmatic Use

To set the block parameter value programmatically, use the set_param (Simulink) function.

To get the block parameter value programmatically, use the get_param (Simulink) function.

Parameter:	SampleTime
Values:	-1 (default) | nonnegative scalar
Data Types:	double

Programmatic Use: Add Block

To add the block programmatically, use the add_block function with one of these library block paths as the source.

"mqttlib/MQTT Client Publish"

Example: add_block("mqttlib/MQTT Client Publish","myModel/myBlock",MakeNameUnique="on")

For information about programmatic access to individual block parameters, see the Programmatic Use section of parameter descriptions on this page.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™. (since R2026b)

Limitation:

This block generates platform-specific code for the host machine's platform only (Windows^®, macOS, Linux^®).

Usage Notes:

When you use password authentication in code generation workflows, provide passwords as environment variables.

• setenv("<MQTTClientName>_password","<yourPassword>") sets the client password.

• setenv("<MQTTClientName>_clientkeypassword","<yourPassword>") sets the client key password.

MQTTClientName is the name that you specify when you add the MQTT client in Simulink^®.

For example, if the client name is MQTTHivemqClient, set the password using setenv("MQTTHivemqClient_password","password123").

Version History

Introduced in R2025a