Short summary

Name	`MQTT_Unsubscribe`
→POU type	→function
Category	more system blocks, MQTT, block with internal error diagnostic
Graphical interface
Available since	version 2.0.17 (for library `System`) and version 4.5.0 of the →runtime system This block is supported for the built-in PLC and →Raspberry Pi.

Functionality

The block unsubscribes an established connection determined by a connection handle from a given topic.

The data is transferred by means of an already existing MQTT broker

(see "Preparing/Realizing data transfer via MQTT")
.

Inputs, outputs, return value

	Identifier	→Data type	Description
Inputs:	`ch`	`DINT`	connection handler (as obtained by the MQTT_Connect block)
Inputs:	`topic`	`STRING`	topic to unsubscribe from
Outputs:	`rc`	`MQTT_RC`	return code of involved MQTT functions as specified in data type `MQTT_RC`

Input EN and output ENO are available when →calling the block. See "Execution control: EN, ENO" for information on input EN and output ENO.

See:

"Blocks for safe logic" whether this block is supported for developing safety-relevant applications.
"Highlighting safe logic in the FBD-editor" for the effects when using the block as a block for safe logic .

Internal error diagnostic for block

The block checks the following error cases and – if they occur – the block sets the output ENO of the block to the value FALSE (or an equivalent):

An internal error occurred while sending a notification to the →runtime system .
The functionality of the block could not be executed.

Example for usage within ST-editor

PROGRAM MQTTSubscribeUnsubscribeExample
	VAR
		connection_handler : DINT := -1;
		return_code : MQTT_RC;
		subscriber_state : MQTT_SUBSCRIBER_STATE := MQTT_SUBSCRIBER_STATE#INVALID;
		subscribed_to_topic : BOOL := FALSE;
		subscribe_flag : BOOL := FALSE;
	END_VAR
	
	subscriber_state := MQTT_GetState(ch := connection_handler, rc => return_code, ENO => ENO);
	
	/* Establish a MQTT connection to a broker */
    IF subscriber_state <> MQTT_SUBSCRIBER_STATE#CONNECTING AND subscriber_state <> MQTT_SUBSCRIBER_STATE#CONNECTED THEN
        connection_handler := MQTT_Connect(address := 'tcp://192.168.1.100:1883', clientId := 'RTS1', rc => return_code, ENO => ENO);
    END_IF;
    
    /* Check if the client is connected to the broker */
    IF subscriber_state = MQTT_SUBSCRIBER_STATE#CONNECTED THEN
    	/* If the subscribe flag is set... */
    	IF subscribe_flag THEN
	    	/* ...make sure the client is subscribed to a specific topic... */
	    	IF NOT(subscribed_to_topic) THEN
	            MQTT_Subscribe(ch := connection_handler, topic := 'example-topic', rc => return_code, ENO => ENO);
	            subscribed_to_topic := return_code = MQTT_RC#OK;
	        ELSE
			    /* ...and publish or receive messages over MQTT inside this block */
	        END_IF;
	    /* If the subscribe flag is NOT set... */
	    ELSE
	    	/* ...and the client is subscribed to a specific topic... */
	    	IF subscribed_to_topic THEN
	    		/* ...unsubscribe the client from this topic */
	    		MQTT_Unsubscribe(ch := connection_handler, topic := 'example-topic', rc => return_code, ENO => ENO);
	    		subscribed_to_topic := NOT(return_code = MQTT_RC#OK);
	        END_IF;
	    END_IF;
    END_IF;
 
END_PROGRAM

When creating your application within the ST-editor, enter a call of a block by typing the text as requested by the syntax or use Content Assist.