/*---------------------------------------------------------------------------------------------------
*	USS message structure.  Information sent by ISC control computer 8 times per second. 				 

*	Ultrasonic sensor minimum period is 46+ milliseconds.  Therefore, the ISC period is
*	scheduled by the tasker at 1/16 seconds (62.5 msec periods).  The time margin (62 msec - 46 msec)
*	gives time for echos to die down.  Reporting to the navigation computer is on 1/8th second
*	intervals (125 msec period), hence two sensors are reported on each 1/8th second reporting interval.
*	ROM is compiled for 2 ultrasonic sensors, then each sensor has new data every reporting interval.
*	ROM is compiled for 4 ultrasonic sensors, then each sensor has new data every other reporting interval.
*   
*	Only one Ultrasonic sensor can be active at a time inorder to avoid interference.  Hence sensors are 
*	activated in sequential order on 62.5 msec periods.  Individual time tags are captured when the
*	sensor is triggered active to begin ranging.  Each count in the time tag is .9765625 milliseconds.
*	Each count in the range measurement is .953687 micro-seconds (16,777,000/16 = 1,048,562 Hz.)

*	The SRF04 Ultrasonic sensor provides an echo pulse proportional to distance.  Multiply the reported
*	range count * 0.953687 to compute the range in micro-seconds (usec).  For example,
*		usec/58  is the distance in inches.
*		usec/148 is the distance in centimeters.
*
*	If nothing is detected then the SRF04 will report a range anyway of about 36,000 usec.
*	If the sensor was not triggered or is absent, the "rangex_count" field will report = 0;
*---------------------------------------------------------------------------------------------------*/
/* ISC to NAV 8 times per second   UltraSonic Sensor					*/
struct USSMSG {				/* Synchronization word				*/
	USHORT unique_word;		/* Word 1, 0xff80 - 0xff first, 0x81 second	*/
	USHORT msgID;			/* Word 2,										*/
	USHORT msglength;		/* Word 3, Number of words in the structure.
								Including unique_word and checksum	*/

	short	us_sensor1_id; 			/* Word 4,  1,2,3,4.	(0 is no sensor)	*/
	unsigned short range1_count;	/* Word 5, 0.953687  microseconds per count.	*/
	ULONG	time1_tag;				/* Word 6, 0.9765625 milliseconds per count	*/

	short	us_sensor2_id; 			/* Word 8,  1,2,3,4.	(0 is no sensor)	*/
	unsigned short range2_count;	/* Word 9,  0.953687  microseconds per count.	*/
	ULONG	time2_tag;				/* Word 10, 0.9765625 milliseconds per count	*/

	USHORT	checksum;	// Word 12.	The method is to sum the entire structure from the unique_word
						// up to, but excluding the checksum word.  All math is performed using
						// signed 16 bit integers with carries, or overflows, ignored.  The sum is 
						// negated.
};
typedef struct USSMSG USSMSGst;