Tom's Life Bits

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===== APRS Telemetry System =====

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;
	while (value >= 100)
	{
		value -= 100;
		count++;
	}
	ax25sendByte(count + 48);

count = 0;
	while (value >= 10)
	{
		value -= 10;
		count++;
	}
	ax25sendByte(count + 48);
	ax25sendByte(value + 48);
	return;
   }

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