Basler Vision Connector Messaging Concept#

The message exchange is based on JSON messages. The following sections outline the minimum content for the message payload.

For ZMQ, the JSON string payload is encoded in a UTF-8 single-frame message.

In MQTT messages, the JSON payload serves as the content of the message to be sent.

Topic Structure#

ZMQ#

When using ZMQ connections, use the expected structure outlined below:

To send commands, use single-frame messages sent to the REQ/REP port.
To stream images, use the device/{DeviceID}/stream topic on the PUB/SUB port.
To receive human-readable application feedback, subscribe to the app/logging topic on the PUB/SUB port.
To receive application events that should be handled by the subscriber, subscribe to topic app/events on the PUB/SUB port.

Message Envelope for ZMQ Messages in the Req/Rep Channel#

In the Basler Vision Connector, messages (request and response) with only one frame are used:

Frame	Field	Description	Type
1	payload	Contains the JSON payload.	Binary string (UTF-8)

MQTT#

For MQTT connections, the topic structure is as follows:

To send commands, publish messages to {mqttRoot}/app/request.
To receive application command responses, subscribe to the {mqttRoot}/app/response topic.
To receive human-readable application feedback, subscribe to the {mqttRoot}/app/logging topic.
To receive events which should be handled by the subscriber, subscribe to the {mqttRoot}/app/events topic.

The message content should be formatted in JSON, employing the message field names as property names for each call, as described in the different "Messaging" topics.

Message Envelope for MQTT Messages#

In MQTT, the JSON messages are written directly into the payload of the MQTT message. No specific envelope is required.