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
• Printable ASCII.

Sensors only speak when spoken to by a data recorder.

1. Starts at 0V.
2. “Break” - wakeup signal - high for >=12mS
3. low (mark) >= 8.33mS
4. address / command / data.