Publications
To publish data on MQTT, the generic Get_Data_Notification command of the Application host object (FFh) shall be used. The command makes it possible to publish either a single ADI value, the values of ADIs that belong to an Assembly Mapping instance, or vendor specific data that is published transparently from the host application. The Get_Publish_Configuration command of the MQTT host object (E2h) gives the possibility to customize some options of the MQTT publish packet per publication. Once the Anybus CompactCom has successfully set up a connection to the configured broker it will be possible to publish data. The publish sequence is described below.
The Anybus CompactCom sends a Get_Data_Notification request to the Application host object once a connection to a broker is active. If the host application has data to publish it can respond to it immediately. If there is currently no data available, the host application can choose to either keep the request to be able to respond immediately when data is available or respond with error code 17h (no data available). In the later case, the Anybus CompactCom will enter a poll mode and periodically poll the host application by sending the request again.
When the application has data to publish, e.g. if data is changed or if someone pushes a button, it responds to the Get_Data_Notification request from the Anybus CompactCom that has been kept from Step 1, or waits for the next request. The response includes the dataset to publish to the broker. The dataset is either a single ADI value, the values of ADIs that belong to an Assembly Mapping instance, or vendor specific data that is published transparently from the host application. Optionally, a timestamp can be included.
The Anybus CompactCom then sends a Get_Publish_Configuration request to the MQTT host object (E2h) to retrieve any defined publication options for this publication.
The application responds with its publication configuration or with an error code, if default options are wanted.
Depending on dataset, the Anybus CompactCom requests more information from the application.
The Anybus CompactCom encodes the dataset using JSON, if the dataset is either a single ADI value or the values of ADIs that belong to an Assembly Mapping instance. Vendor specific data is not encoded and is published transparently.
The Anybus CompactCom builds the MQTT message and publishes it to the MQTT broker. If the retain bit is set, the message will be saved in the MQTT broker for future subscribers to collect.
The figure below shows an example of a publishing sequence with the numbers from the sequence above included. The order of 3 and 5 may be different depending on the Anybus CompactCom implementation.
When the Anybus CompactCom has published a message to the broker, it repeats the sequence and sends a new Get_Data_Notification request. This sequence is repeated as long as the Anybus CompactCom is connected to the broker.
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