HB6270IR:  Files to support IR beacons between two robots.

OVERVIEW:  Each of two robots has an IR beacon plugged into the HB's
IR output port.  One robot transmits a 100 Hz signal over IR; the
other one, 125 Hz.  Each robot has up to four IR detectors that
indicate if it sees the signal transmitted by the other robot.

IR BEACON is plugged into the HB's IR port.  The following functions
are used for transmission:

    ir_transmit_on()			turns on IR transmission.
    ir_transmit_off()			turns off transmission.
    set_ir_transmit_frequency(int f)	sets IR transmission frequency.
					should be either 100 or 125.


IR RECEIVERS are plugged into the HB's digital inputs, ports 12
through 15.  The following functions are used to enable reception:

    ir_receive_on()			enables IR reception.
    ir_receive_off()			disables IR reception.
    set_ir_receive_frequency(int f)	sets IR reception frequency.
					should be either 125 or 100.
    ir_counts(int port)			returns number of consecutive
					cycles of successful IR
					reception for given port (12
					15).  0 indicates no
					reception.  255 indicates
					about 2 seconds of continuous
					reception.  Any number higher
					than 5 should be considered
					solid detection. 

If a robot is transmitting at one frequency, it should be detecting at
the other frequency.  E.g.:

void robot1()
{
  set_ir_transmit_frequency(125);
  ir_transmit_on();
  set_ir_receive_frequency(100);
  ir_receive_on();
}

