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BUSNET :: Level 2

BUSnet level 2 (transport) is responsible for ensuring a node's data is placed correctly onto the bus and conversely data from other nodes is received from the bus with no undetected errors.

BUSnet level 2 uses a "publishing/subscribing" model. In a nutshell a node's application simply blurts its data onto the bus and all other node applications read that data. Whether or not they do anything with it is of no concern to the level 2 protocol, it's up to software running at a higher level such as the BUSnet level 3 protocol (when developed).

However it's a simple matter to design your own protocol, just treat the network as a virtual I2C port and send/receive commands according to your requirements.

Note that all this is dealt with by the PIPE chip. As an application programmer you don't need to know anything about frame formats etc.


    Frame format    

The frame format is still under review but at present it is as follows.


SOF – A sequence of 10 consecutive 0s that indicates a start of frame.

UNI – The Unique Node Identifier of the originating node.

SEQUENCE – This sequence number of the frame. This and the UNI field uniquely identify the source of the frame.

UFI – The frame’s Unique Frame Identifier.

T – Timestamp flag, if set the 3-byte timestamp is included in the frame.

TYPE – Frame type.

TTL – Time To Live. This field is used to stop orphan frames from circulating for all time.

LENGTH – This field indicates the number of bytes in the payload. Details in the following section.

PAYLOAD – Between 1 and 256 bytes as per the preceding LENGTH field.

TIMESTAMP – Timestamp. An optional 15-bit counter indicating the millisecond that the event occurred. Note that level 3 always gets a timestamp despite it being optional at level 2.

L – A flag that indicates if the timestamp is locally generated or sent by the originating node.

CRC – A 16-bit CRC calculated on all preceding bytes, algorithm TBD.

EOF – A sequence of 9 consecutive 1s that acts as a frame separator.

LEVEL 3 FRAME – Data packet delivered to level 3, see the Level 3 Protocol chapter.



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