MQTT_CONNECT "192.168.1.123:1883", 10	

1. The URL_string can be either the domain name or the IP address of the MQTT broker. Note: if you use SSL, then the domain name is also the common name contained in the CA cert. If the broker does not have a domain name and only has an IP address then you should not enable broker's common name verify (see MQTT_CONFIG 10,"0") or else it may fail the verification of the CA cert.
2. Use the MQTT_STATUS() function to check if the connection is sucessful and to check if the PLC is still connected to the MQTT broker. MQTT_STATUS(1) will return a '1' indicating that the operation is successful. MQTT_STATUS(2) will return a '1' when the PLC is connected to the broker.
3. The callback Cust_Fn is the interrupt service routine that the operating system will invoke when a new MQTT message arrives. The callback Cust_Fn can then identify the topic and process the data using the MQTT_TOPIC() and MQTT_DATA() functions.

Note:

1. This command can be used for both non-encrypted and encrypted (SSL) connection to the MQTT broker. By default the MQTT broker listens on port #1883 for non-encrypted connection and on port #8883 for encrypted connection. If SSL connection is used then you must define the CA cert filename using the command: MQTT_CONFIG 9, "ca_cert_filename". The PLC will check and if CA cert filename is an empty string it defaults to non-SSL connection, otherwise it will attempt to connect using SSL encryption
2. SSL connection (default port #8883) with both server and client certificates verification takes the longest (up to several seconds) to complete the connection due to the need to exchange certificates, encrypt and decrypt messages during the connection negotiation.
3. SSL connection with only server side verification takes slightly shorter time to complete the connection. But non-SSL communication (default port # 1883) is both the fastest and much less complicated to get a successful connection. Hence, If your MQTT broker and PLCs are all residing on a private local area network it may be unnecessary to enable SSL encryption
4. By default, the PLC will connect to the MQTT broker using a "clean session" - what it means is that each time the PLC connects to the broker it is treated as a new connection and none of its previous connection preferences (such as subscribed topics, LWT messages etc) are remember. You can change the connection session to "persistent" by running the MQTT_CONFIG 1,"1" command before the PLC connects to the broker and the broker will remember the session even after the PLC disconnects and then reconnects to the broker.
5. i-TRiLOGI 7.5 simulator is now able to actually connect to a MQTT broker like the real PLC. So you can fully test the MQTT operation using just the simulator without the need to transfer the program to the PLC first.