ABOUT SDI-12

SDI-12 stands for “Serial Data Interface at 1200 baud” and is a common protocol for interconnection of environmental sensors and dataloggers. The SDI12 standard has been developed and maintained by the SDI12 support group which is accessible at www.sdi-12.org. The SDI12 standard is a powerful protocol for connecting sensors offering a lot of advantages over analog measurements or other standardized or proprietary serial protocols.

Simple 3 wire interface

Measurement Science maintain this web page to provide:

• Detailed technical reference and support material for people wanting to learn more about the protocol, to add to the efforts of the SDI-12 Support Group
• Resources relating to SDI12 based products that we have developed to suit environmental applications

ADVANTAGES OF SDI-12 OVER OTHER METHODS

• 3-wire interface. A simple, easy to understand and configure, 3 wire interface, consisting of Power (12V) + Ground + Data. In general, the color standard is Red for power, Black for ground, that the Data wire is the "other" color, commonly white, green or yellow (depending on the manufacturer)
• Addressable Bus. Multiple sensors can be added to the same 3-wire bus without conflict, by using a unique SDI12 address for each sensor. The SDI12 standard includes the command to easily change the address of any sensor (using aAb!). The address range can typically be "0” to “9”, “A” to “Z”, or “a” to “z”, giving a total of 62 unique addresses. To connect multiple sensors, wire all the power together, all the ground together, and all the data together
• Long cable connections. With a low 1200 baud rate, the protocol can support long cable runs. A slow baud rate is sufficient for environmental applications because the parameters are relatively slowly changing, compared to industrial applications
• Remote terminal. Most measurement systems capable of reading SDI12 sensors (i.e. Campbell Scientific dataloggers such as the CR1000) feature SDI12 transparent mode which allows direct connection to the sensor via the datalogger interface. When the datalogger is connected remotely, such as with a cellular or satellite modem, this is a powerful tool to connect directly to the sensor itself and perform diagnostic tests
• Internal calibration. Instead of analog 4-20mA or 0-1V sensors often requiring individual calibration (such as 4mA = 0m H2Oand 20mA = 10m H2O), an SDI12 sensor provides the calibrated measurement directly, without calibration coefficients in the program. This simplifies the program and often eliminates the need for station specific programs
• Additional parameters. To provide temperature compensation, environmental sensors typically include an internal temperature sensor that is not usually wired as a separate analog output as this requires significant additional wiring. With SDI12 sensors however, this is often provided as part of the standard measurements, allowing more detailed data than analog sensors provide

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