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Let's examine the structure of the messages exchanged between masters and remotes. Basic serial telemetry protocols, like TBOS, are byte-oriented, with a single byte exchanged to communicate. Expanded serial telemetry protocols, like TABS, are packet oriented with packets of bytes exchanged to communicate. The packets contain header, data and checksum bytes. DNP3 is also packet oriented and uses the packet structure (element sizes in bits) shown in the illustrated figure below.
DNP3 Packet Structure
The Master sends a Read request for an object or objects and the Remote's response contains the requested information if available. The Master sends an Operate command to produce the output actions associated with the selected object reference. The Remote sends an Unsolicited Message when a specific event occurs.
The preceding figure shows the message packet format. The DNP3 application service data unit (ASDU) is worthy of special note for the clever content adjustment that is controlled by the qualifier and indexSize fields. This design makes application data available in an impressively flexible number of configurations or omitted all together if desired.