Tom's Life Bits

APRS TELEMETRY FORMAT: The on-air packet telemetry format is as follows:

T#sss,111,222,333,444,555,xxxxxxxx  where sss is the serial number
                                    followed by the five 3 digit analog
                                    values and the eight binary values.

Each telemetry value is actually a decimal number between 000 and 255. The user can adjust his sensors to meaningful values, or the telemetry equations can be modified on receipt. Usually just a a simple 10k and 2.4k resistor divider is used on channel 1 to give you battery voltage in tenths. So a reading of 138 would mean 13.8 volts. It is easy to make APRS display any other quadratic telemetry equation too.

Some source code fragments to achieve this:

 void ax25sendASCIIebyte(unsigned short value)
 {
static char count;
if (value > 999) value = 999;
count = 0;
      // first get the MSD (hundreths)
while (value >= 100)
{
	value -= 100;
	count++;
}
ax25sendByte(count + 48);
count = 0;
while (value >= 10)
{
	value -= 10;
	count++;
}
ax25sendByte(count + 48);
ax25sendByte(value + 48);
return;
 }