MqttPublish – Publish MQTT message

Block SymbolLicensing group: MQTT

Function Description

This function block depends on the MQTT driver. Please read the MQTTDrv manual [10] before use.

The MqttPublish block publishes messages to an MQTT broker through the connection established by the MQTTDrv driver.

The first parameter is the topic the block will publish the messages to. MQTT delivers Application Messages according to the Quality of Service (QoS) levels. Use the QoS parameter to set a different Quality of Service level. See the MQTT specification [11] for more details.

If the RETAIN parameter is set a RETAIN flag will be set on the outgoing PUBLISH Control Packet. See the MQTT specification [11] for more details.

The defBuffSize parameter can be used to optimize the memory usage of the block. It states the amount of the statically allocated memory for the inner buffer for the outgoing messages. If the value is unnecessarily large the memory is being wasted. On the other hand if the value of the parameter is too small it leads to frequent dynamic memory allocations which can be time consuming.

The message to be published is constructed from the value input signal. The value input signal is expected to be a string. If it is not a string it will be converted automatically. To request a message to be published in the current period set the RUN flag to on. The BUSY flag is on if the block has a pending request and waits for a response from a broker. When the response is received in the current cycle the DONE flag is set to on.

Inputs

value	Input signal	String
RUN	Enable execution	Bool

Parameters

topic	MQTT topic	String
QoS	Quality of Service $⊙$ 1	Long (I32)
	1 .... QoS0 (At most once) 2 .... QoS1 (At least once) 3 .... QoS2 (Exactly once)
RETAIN	Retain last message $⊙$ on	Bool
defBuffSize	Default buffer size $↓$ 1 $⊙$ 2048	Long (I32)

Outputs

BUSY	Busy flag	Bool
DONE	Indicator of finished transaction	Bool
errId	Error code	Error

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