Designed to be light weight, open, and simple, MQTT is a subscriber/publisher messaging transport protocol that is considered a great solution for applications where small code footprint is required and/or network bandwidth is scarce. It is particularly suitable for continuous monitoring of sensory data such as temperature, pressure, water level, energy monitoring…etc.
How HMI fits in
HMI processes data from PLC and publishes messages to an MQTT broker, which will handle message delivery to the subscribers. In this manner, message publishing is possible even when the HMI is positioned behind firewalls and access to HMI is difficult.
Alternatively, MQTT messages can be published internally to a built-in MQTT broker. That is, an external broker is not necessary; one can use an MQTT client to subscribe directly to the MQTT broker inside the HMI and receive message updates!! This scheme can be realized even remotely as long as the MQTT client can connect directly to HMI, such as with VPN or EasyAccess 2.0.