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====== SDI-12 ======

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.

[[http://www.sdi-12.org/current_specification/SDI-12%20Specification%201.4%20February%2020%202023.pdf|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.

===== Protocol =====

  - Starts at 0V.
  - "Break" - wakeup signal - high for >=12mS
  - low (mark) >= 8.33mS
  - address / command / data.