Short summary

Name	`MQTT_Publish`
→POU type	→function
Category	more system blocks, MQTT
Graphical interface
Available since	version 1.28.0 (for logi.CAD 3) and version 2.3.1301 of the →runtime system – initial variant version 1.108.0 (for logi.CAD 3) and version 3.18.0 of the →runtime system – enhancement: data type `STRING` for existing input `topic` (instead of `STRING[255]`); data type `MQTT_RC` for output `rc` (instead of `DINT`); data type `MQTT_SUBSCRIBER_STATE` for return value (instead of `SINT`) This block is supported for →Raspberry Pi.

Functionality

The block publishes a message for a given topic via the connection that is specified by the connection handler. The messages are published asynchronously.

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

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

Inputs, outputs

	Identifier	→Data type	Description
Inputs:	`ch`	`DINT`	connection handler (as obtained by the MQTT_Connect block)
	`topic`	`STRING`	topic for the message
	`payload`	`REF_TO BYTE`	payload of the message
	`len`	`DINT`	length of the payload to publish
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 .

Example for usage within ST-editor

Program with selected calls of MQTT blocks

PROGRAM Program1
    VAR
        ioImageLocal : ARRAY[0..1023] OF BYTE;
        ioImageLocalLen : DINT := 2;
        ch : DINT := -1;
        cnt : DINT := 0;
        rc : MQTT_RC;
        state : MQTT_SUBSCRIBER_STATE := MQTT_SUBSCRIBER_STATE#INVALID;
    END_VAR
    
    state := MQTT_GetState(ch := ch, rc => rc, ENO => ENO);
    
    /* make sure MQTT connection is established and messages are received */
    IF state <> MQTT_SUBSCRIBER_STATE#CONNECTING AND state <> MQTT_SUBSCRIBER_STATE#CONNECTED THEN
        ch := MQTT_Connect(address := '192.168.1.107', clientId := 'RTS1', rc => rc, ENO => ENO);
    END_IF;
    IF ENO AND state = MQTT_SUBSCRIBER_STATE#CONNECTED THEN        
        /* prepare data to be transferred over MQTT */
        ioImageLocal[0] := 16#CA;
        ioImageLocal[1] := 16#FE;
        /* send updated io image to remote */
        MQTT_Publish(ch := ch, topic := 'sample_topic',
                        payload := REF(ioImageLocal[0]),
                        len := ioImageLocalLen,
                        rc => rc);
    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.