Communications Substrate
We felt that a minimal, lightweight communications protocol was most
appropriate to this project. Consequently, rather than using the monitor
supplied with the Handy Board, we wrote our own protocol.
The robot code is written in Interactive-C. The host code, like the rest of
Altaira, is written in C++.
Communication From Robot to Host
Whenever the robot is turned on, it continuously reads its five analog
and eight digital sensors and transmits the raw data to the host. The
packet format used is:
In order to maintain synchronization, each six byte packet is preceded by
a start-packet character (0xbb), and followed by an even
vertical parity byte.
The sensors (an0 through an4) are
in order from left to right. The digital sensor value is simply the
value read from the digital sensor port.
Communication From Host to Robot
The host sends one byte to the robot on each execution cycle, containing
commands for the four motors (only two of the motors are actually used,
but since the minimum unit of tranmission is one byte, transmitting the
commands for all four takes no extra time). The byte format is
that used by the Handy Board's
hardware motor port. The left motor is motor 0; the right is motor 1.
Synchronization
All synchronization is maintained by the host. On each execution cycle one
sensor packet is read, and one motor control byte is written. The host's input
queue is checked, and if it falls a packet behind the packed is dropped.
If either the robot or the host does not receive data from the other end for
a short period of time (on the order of half a second), it concludes the link
is down. The host will not perform execution cycles as long as it believes
the link is down; the robot will shut its motors off if it believes the link
is down. In spite of this, the robot will continue to attempt to send sensor
packets to the host, so communications will be re-established immediately if
the link becomes live again.
Robot Display
The Handy Board's LCD display is used to display the current values of the five
sensors and the link status.