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Physical Level
VSCP over RS-232
Level I
Preliminary
A byte stuffing binary interface is used to talk to the module through the serial interface.
The protocol is created for VSCP Level I events but can be used for other purposes also, as the 16 byte data content indicates. In this case the class and type bytes can be used freely by the implementor.
frame formats
| [DLE] |
| [STX] |
| Operation 0 = No operation 1 = Level I event to/from node. 2 = Level II event to/from node. 3 = Poll for event. 4 = No events to send. 5 = CAN Frame. 10 = Reserved. 249 = Sent ACK. 250 = Sent NACK. 251 = ACK. 252 = NACK. 253 = error. 254 = Command Reply. 255 = Command. |
| Flags + bit nine of class bit 0-4 - Number of databytes bit 5 - Bit nine of Class. bit 6-7 - Reserved and should be set to 0. |
| Channel |
| Sequence number |
| Class (Lower 8 bits). |
| Type |
| Data byte(s) 0 - 15 |
| Frame checksum (counted from address (low 8-bits of it). XOR checksum is used. |
| [DLE] |
| [ETX] |
There can be 0 to 16 databytes.
The checksum is calculated from, and including, flags up to the last databyte.
The channel byte can be used to logically separate different channels on the same physical line. Typically this can also be a serial line with many nodes which are polled for data.
The sequence number byte marks a number for a sequence of a special frame. For instance four frame are send after each other and they will get different counter numbers. The NACK/ACKS that are responded from the node use the same counter values to separate the frames from others. Can, for example, be used to implement a sliding window protocol (http://en.wikipedia.org/wiki/Sliding_window).
As noted there can be two types of ACK's/NACK's one (251/252) is a general ACK/NACK that is the response when a frame is received and put in a buffer. This is generally all that is needed. The other version (249/250) can optionally be sent when the frame is actually sent on the interface.
Command frames have the following format
| [DLE] |
| [STX] |
| 255 = Command. |
| Flags Bit 7-5 reserved. bit 0-4 - Number of databytes. |
| Channel |
| Sequence number |
| Command MSB |
| Command LSB |
| Optional command Data byte(s) 0-15 |
| Frame checksum |
| [DLE] |
| [ETX] |
There can be 0 to 16 argument bytes.
When a command is sent their should always be a response. This response could be an ACK/NACK/Response/Error frame so all types should be expected.
Error frames have the following format
| [DLE] |
| [STX] |
| 253 = error. |
| Flags Bit 7-5 reserved. bit 0-4 - Number of databytes. |
| Channel |
| Sequence number |
| Error code MSB |
| Error code LSB |
| Optional error data byte(s) 0-15 |
| Frame checksum |
| [DLE] |
| [ETX] |
There can be 0 to 16 argument bytes.
Command Reply frames have the following format
| [DLE] |
| [STX] |
| 254 = response. |
| Flags Bit 7-5 reserved. bit 0-4 - Number of databytes. |
| Channel |
| Sequence number |
| Command Reply code MSB |
| Command Reply code LSB |
| Optional error data byte(s) 0-15 |
| Frame checksum |
| [DLE] |
| [ETX] |
There can be 0 to 16 argument bytes.
ACK frame have the following format
| [DLE] |
| [STX] |
| 251 = ACK. |
| Channel |
| Sequence number |
| Frame checksum |
| [DLE] |
| [ETX] |
NACK frame have the following format
| [DLE] |
| [STX] |
| 252 = NACK. |
| Channel |
| Sequence number |
| Frame checksum |
| [DLE] |
| [ETX] |
Special characters
The DLE character is sent as [DLE][DLE]
[DLE] = 0x10 [STX] = 0x02 [ETX] = 0x03
Sending CAN frames with the protocol
For generic CAN frames the operation byte is set to 5
The first four databytes form the CAN ID.
The four last databytes can be a timestamp (big endian).
Bit 31 (offset 0) of the id is Extended frame.
Bit 30 is remote frame.
Bit 29 is error frame.
Command codes
Command codes and data format is decided by the implementor.
Response codes
Response codes and data format is decided by the implementor.
Error codes
Error codes and data format is decided by the implementor.
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