Communications protocol:

serial data, 57600 baud, 8N1 format

bytes 0-223 for data
bytes 224-255 for control


All messages consist of optional data bytes, followed by a control
byte to complete the message.  Only bytes 0 to 223 may be used for
data.  Bytes 224 to 255 are reserved for control bytes.

Control byte 224 is a "no operation" command.  All devices must
discard any stored data.  So 224 may be transmitted first before
data, to assure that the message does not unintentionally include
any data bytes that may have been present before.


command						turn-around req'd
-------						-----------------
224: no operation				no
225: wheel positions (not implemented)		no
226: sonar ping					no
227: sonar read request				yes
228: sonar read reply				yes
229: reboot to bootloader			no
230: set motor					no
231: motor status request			yes
232: motor status reply				yes
233: set led					no

Some messages require waiting a brief turn-around delay.  This allows time
for one devices rs-485 transmitter to disable before another device enables
its transmitter and begins driving the line.




No Operation (224):
-----------------

Wheel position message (225):
----------------------------
This command is not implemented


Sonar ping message (226):
------------------------
This command instructs one or more sonar sensors to transmit a ping.  The data
is a list of all sensor addresses that should ping.  No reply is sent.  After
20 ms, each of the sonar sensors may be querried.


Sonar read request (227):
------------------------
This command requests the last sonar reading from a single sensor.  A single
data byte specifies the address of the sensor which is to reply with its
last reading.


Sonar read reply (228):
----------------------
This message is sent by a sonar sensor in response to the read request.  Two
data bytes are transmitted, the number of inches measured, and a serial number.
The serial number is from 1 to 255 (todo: bug... count shouldn't go over 223).
The count may be compared to the previous reply's count to determine if this
is truly a new measurement, or simply the same value read last time.


Reboot To Bootloader (229):
--------------------------
This command instructs all motor control boards to reboot to their bootloader.
Presumably a new firmware image will be transmitted within 10 seconds.  This
command requires 3 data bytes, which must be 195, 85, 19.


Set Motor (230):
---------------
This command sets new parameters for all 4 motors on a motor control board.
Five data bytes are required, the board's address, followed by 4 new settings.
198 disables the motor.  199 is "no change".  0 and 64 stop the motor (hold
its current position).  1 to 63 set clockwise motion (1=slow, 63=fast), and
65 to 127 set counter clockwise motion (65=slow, 127=fast).  All other values
are undefined and should not be sent.


Motor Status Request (231):
--------------------------
The command requests a motor control board to transmit the status of its
four motors.  One data byte is sent, for the address of the board to respond.


Motor Status Reply (232):
------------------------
This is the reply to the motor status request.
Five data bytes are sent.  Additional bytes may be added in the
future, so receivers should allow for more than 5 bytes but only parse the
first 5.  Byte 1 is the enable/disable status for all 4 motors.  Bytes 2
to 5 are the current angular position.


Set LEDS (233):
--------------
Set the 8 LEDs on a motor control board.  Byte 1 is the board address
in bits 0 to 5, bit 6 is the MSB for the LEDS.  Byte 2 is the low 7 bits
to appear on the LEDs.



Sound play message:
------------------
to be defined


Sound stop message:
------------------
to be defined


Poweroff message:
----------------
(not implemented)