SDI‑12 was originally developed in 1988 for microprocessor-based environmental sensors. In 1992, the SDI‑12 documentation was refined by NR Systems, under contract with the U.S. Geological Survey. The intention was to provide a common sensor interface for environmental data collection equipment. The SDI‑12 technical committee continues to maintain the standard.
Electrically, SDI‑12 uses a single 5 V signal line at a fixed data rate of 1200 bps. While slow and not as electrically robust as RS‑485, the protocol has been used widely in its niche application due to the stability and universal compatibility defined in the protocol. The protocol also allows for very lengthy measurement processes that can take up to 30 seconds to complete.
The master-slave architecture can have 10 sensors or more connected to the same SDI‑12 port.
What do sensors connect to?
Many data loggers include an SDI‑12 interface. Dyacon THP‑2 and WSD‑2 can be used with SDI‑12 data loggers.
NR Systems maintains a tool for development and validation of SDI‑12 sensors. The accompanying software includes some data logging capability, but the cost and data format make this solution largely impractical for most data logging applications. A new release of the software will improve data logging capability, but the release date is not known.
A number of SDI‑12 to USB or RS‑232 adapters are available. None of these have been evaluated, so no recommendations can be provided at this time.