Serial Port to CAN bus interface
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This device provides 1 CAN port to any computer with a serial port. The serial
port speed is set to 230.4kBit/s, 8n1. The CAN bus sends and receives at
125kBit/s.

CAN messages to and from the controller are formatted in the following format:

* a preamble of 4 bytes: 0xff, 0xff, 0x00, 0x00
* the message id, 4 bytes, MSB first (if you are sending messages with 11 bit
  ids, just set the leading bits to 0, for 29 bit ids set the leading 3 bits
  to 0)
* 1 byte for extended and rtr flags, where:
  * bit 0 is set for extended
  * bit 1 is set for rtr
  * e.g. transmit 0x01 to set the extended flag, but not the rtr flag
* 1 byte for the length of the message (0-8)
* 8 bytes of message content, fill with 0x00 for messages shorter then 8

Example: an extended message with the id 0x01234567, the length 6 and the word
'foobar' as payload:
    preamble:   0xff 0xff 0x00 0x00
    id:         0x01 0x23 0x45 0x67
    extended:   0x01
    length:     0x06
    content:    0x66 0x6f 0x6f 0x62 0x61 0x72 0x00 0x00


The device may respond with a pseudo-message with an id > 2^29 in case of the
following errors:
* 0xff 0xff 0xff 0x62('b')  send-buffer is full
  * this could be a bus error (no device acknowledged the messages you sent)
  * otherwise just wait a moment and try again
* 0xff 0xff 0xff 0x6d('m')  malformed message
  * the message you were trying to send may have contained an invalid id
* 0xff 0xff 0xff 0x65('e')  other error
  * couldn't send the message

In case of an error the message length is always 4 and contains the id of the
message that couldn't be sent.