SDI-12 is a serial data standard used for measurement instruments, particularly environmental sensors. I have seen it used for soil moisture sensors and weather instruments.
Specification
One data wire plus 12V power and ground.
Power 12V nom, (9.6V to 16V)
1200 baud
5 V signal.
Max 0.5A sensor draw.
Mark 0V (-0.5 to 1.0V)
Space 5V (3.5 to 5.5V)
I think it idles low (0V, logic 1).
1 start bit
7 data bits, least significant bit transmitted first
1 parity bit, even parity
1 stop bit
-
Sensors only speak when spoken to by a data recorder.