MQTT Publish

Publish messages to MQTT broker on specified topic

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Libraries:
Raspberry Pi Blockset / Internet Of Things

Description

The MQTT Publish block publishes messages from your hardware (publisher) to the Message Queuing Telemetry Transport (MQTT) broker on the specified topic.

The block has a Message input port and a Status output port. The Message port accepts the message to be published to the broker. The Status port outputs the status of the publish request. The string topic/level on the block icon is specified by the Topic parameter.

For more information on MQTT, see Publish MQTT Messages and Subscribe to Message Topics.

Note

• The MQTT Publish block supports MQTT only over TCP/IP sockets.

• To generate code C/C++ code for this block, you must have an Embedded Coder^® license.

Examples

Publish and Subscribe to Messages on ThingSpeak Using MQTT Blocks on Raspberry Pi

Use Raspberry Pi® Blockset to publish a message to a topic from Raspberry Pi in the ThingSpeak™ MQTT broker. This example also shows how to subscribe to a topic and receive a message from the ThingSpeak MQTT broker to Raspberry Pi. For more information on MQTT protocol, see MQTT Basics (ThingSpeak) and Publish MQTT Messages and Subscribe to Message Topics. In this example, ThingSpeak is the MQTT broker and Raspberry Pi board is the MQTT client (publisher and subscriber).

• Open Model

Ports

Input

Message — Message to be published

vector

Specify the message you want the publisher to publish to the MQTT broker as an array of ASCII characters of type uint8 or string.

Example: [0,3] or Hello

Data Types: uint8 | string

Output

Status — Status code from MQTT broker

scalar

The status code issued by the MQTT broker in response to the publish request. If the broker accepts the message for publishing, the port outputs 0. Any nonzero output indicates that the broker has rejected the message. A message can be rejected for various reasons, such as an incorrect user name or password or an unacceptable protocol version. For more information on status codes from MQTT brokers, see MQTT Status Codes.

Data Types: int8

Parameters

Topic — Topic of the message

topic/level (default) | any valid topic name

Specify a topic on which the message is to be published. A topic can have a maximum of 128 characters. For information on topics and its structure, see Topics in MQTT.

QoS — Levels of QoS

0 (default) | 1 | 2

Quality of Service (QoS) defines the reliability of the message delivery process in MQTT. MQTT provides three QoS levels for message delivery: QoS 0, QoS 1, and QoS 2. The MQTT broker that you are using might not support all three levels of QoS. For example, ThingSpeak™ MQTT supports only QoS 0.

Select a QoS level from the list:

• 0: The publisher sends the message to the MQTT broker once. The broker does not send an acknowledgment of the receipt.

• 1: The publisher sends the message to the MQTT broker at least once. The publisher resends the message if it does not receive an acknowledgment from the broker.

• 2: The publisher sends the message to the MQTT broker exactly once. The publisher and broker exchange acknowledgments to ensure that the message is received only once.

For more information on QoS levels, see Levels of QoS in MQTT.

Retain message — Option to send message to new subscribers

off (default) | on

The Retain message parameter controls how new subscriptions are handled:

• on –– When a new subscription is made to a topic, the broker identifies the message in its database with the matching topic name and sends the message immediately.

  Note

  The MQTT broker retains only the last known message published on a topic.

• off –– When a new subscription is made to a topic, the subscriber must wait until the publisher sends the next message.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

To generate code C/C++ code for this block, you must have an Embedded Coder license. For more information on selecting the right product stack according to your development goals, see Product Stack for Raspberry Pi Blockset.

Version History

Introduced in R2018b