Communication by the MQTT communication protocol

The MQTT technology allows IoT devices to communicate via MQTT broker.
The MQTT (MQ Telemetry Transport) is a simple "Internet of Things" communication protocol. It is based on passing messages between clients through the central server – broker.

The client can be of the Publisher type - sending their messages to defined topic (address, topic). The publisher can be e.g. represented by a sensor or meter.

On the contrary a Subscriber subscribes to receive values from defined topics in order to process such values after receipt.

The client can be a Publisher and Subscriber simultaneously.

During the login sequence, each client is identified by "ClientID" and then optionally by "Username" and "Password". The SSL/TSL support is not implemented so far in the PROMOTIC system.
MQTT protocol defines three levels of message confirmation QoS (Quality of Service)
QoS 0 - the message is sent without confirmation and the delivery is not guaranteed (at-most-once).
QoS 1 - the message is delivered at least once.
QoS 2 - each message is delivered exactly once.
Client of the Publisher type can send data also with Retained attribute (0/1 = not retained/retained data on the brokeru) and with DUP attribute (0/1 = not repeated/repeated sent messages)
Based on the communication encryption level, the MQTT protocol defines usage of 3 network ports:
1883 - (supported) unencrypted transmission
8883 - (not supported) SSL/TLS encrypted transmission
8884 - (not supported) encrypted transmission + SSL/TLS client certificate + the client must provide also the authentication cerificate published by broker

The PROMOTIC system so far does not support:

encrypted transmissions - only port 1883
diacritics in sent and received text messages - ANSII only

Communication of PROMOTIC application with MQTT broker

The logged-in client is both Publisher and Subscriber. The Subscriber client subscribes for receiving defined topic address on the broker. The Publisher client sends its data to this address to be received by Subscriber.

For reading the data it is handy to use the preconfiguration:

Pm9.00.03: New preconfiguration using the PmChar - Driver for communication by user defined ASCII/BIN protocol communication driver.

The client can be of Subscriber or Publisher type (i.e. the preconfiguration can do both reading and writing).

At this moment the communication does not support encryption.

PROMOTIC 9.0.4 SCADA system documentation - MICROSYS, spol. s r.o.

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