Callback Function Declarations - ProSoft Technology MVI56E-LDM-MQTT Quick Start Manual

MVI56E-LDM-MQTT ♦ MQ Telemetry Transport
ControlLogix® Platform

10.2.2 Callback Function Declarations

Functions - Callbacks, called by MQTT-LDM Library during runtime, should be
implemented by the user application. The sample application provides a default
implementation of the callback functions.
int mqtt_ldm_read_value(plc_tag* tag, plc_value* value);

Called to read the value of a tag from the PLC.

The first input argument, tag, is the tag name to be read from the PLC.

The value read from the PLC is returned in the output argument value.

If successful, the read value is returned in the output argument value and the
function returns 0.
int mqtt_ldm_write_value(plc_tag* tag, plc_value* value);

Called to write a new value to a tag.

The first input argument, tag, is the tag name to be written to the PLC.

The input argument value is the value to be written to the PLC tag.

New values are received from MQTT Broker by subscribing to specified
subscription topics in the configuration file.
int mqtt_ldm_is_connected_to_plc(void);

If the module is connected to the PLC, it returns 1. Otherwise, it returns 0.
int mqtt_ldm_get_status(char is_verbose, char** buffer, uint16_t max_size);

This function is used to get the current status of the communication with the PLC.

If the input argument is_verbose is not 0, then more detailed information is
returned.

When argument is_verbose has a non-zero value, sample application mqtt-ldm-
sample-app-mvi56e returns content of log file in the buffer.

The result is copied into the memory buffer. It may be pre-allocated by the caller.
In this case, its size is passed in the max_size argument. If the buffer points to
NULL, then max_size can still be used to limit the size of returned text.

If successful, return 0.
Examples of using of these functions can be found in the source code of the sample
application.
ProSoft Technology, Inc.
MQTT-LDM Library
Quick Start Guide
Page 50 of 55