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Protocol Syntax
The protocol is used to provide a consistent method by which serial communication is sent/received on the boat. It begins and ends with a standard header and footer and contains a payload of varying size dependent upon the function of the message (command) being sent.
The protocol assumes the computer (onboard or shoreside) is issuing a request to the microcontroller. In this way, the computer is able to control the flow and rate of information transfer.
The structure of a message is as follows:
| Start Indicator | Command Number | Payload | CRC16 |
|---|---|---|---|
0xAA |
0x00 to 0xFF
|
Varying size |
0x0000 to 0xFFFF
|
Each message begins with the same byte, always 0xAA.
The type of message being sent is indicated by the command number. Each microcontroller has a block of commands reserved exclusively for it. This provides further consistency and a means of easily adding commands.
| Block Start | Block End | Microcontroller |
|---|---|---|
0x00 |
0x0F |
Wireless Communications |
0x10 |
0x1F |
Propulsion |
0x20 |
0x2F |
|
0x30 |
0x3F |
|
0x40 |
0x4F |
|
0x50 |
0x5F |
|
0x60 |
0x6F |
|
0x70 |
0x7F |
|
0x80 |
0x8F |
|
0x90 |
0x9F |
|
0xA0 |
0xAF |
|
0xB0 |
0xBF |
|
0xC0 |
0xCF |
|
0xD0 |
0xDF |
|
0xE0 |
0xEF |
|
0xF0 |
0xFF |
The payload of each message is dependent upon the command issued. However, each command has a specific payload length associated with both the request and the response message.
The CRC16 (a 16-bit Cyclic Redundancy Check) is a method by which the validity of a serial message can be verified. It is a mathematical equation applied to the entire message (except the CRC itself) by the sender and is then verified by the receiver. If there is a discrepancy then it is known that the message is corrupted.
The CRC-CCITT (XMODEM) is the particular algorithm used.
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