×Latest blog post: Understand the advantages/disadvantages of SDI12 compared to Modbus and NMEA-0183

What makes SDI12 so useful?

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 setting the SDI12 address of each instrument to a unique number on the bus.  The SDI12 standard includes the command to easily change the address of any sensor, of the format:

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

Because the protocol uses a low baud rate (1200 baud), the protocol can support long cable runs.  A slow baud rate is fine because the measurements are small and efficient

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
Remote data, connected.

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About

SDI-12 stands for serial data interface at 1200 baud, and it a commonly used protocol for connection environmental sensors and measurement systems or dataloggers.  SDI12.com is an information resource and online store focused on everything related to the SDI12 measurement standard.  The SDI12 (or SDI-12) standard has been developed and maintained by the SDI12 support group which is accessible here: www.sdi-12.org.  The support group focused on the development of the standard. Companies manufacturing SDI12 sensors may become members.

The SDI12 standard is a powerful protocol for connecting sensors offering a lot of advantages over analog measurements or other standardardized or proprietary serial protocols.  We have a blog post on this topic.  For those interested in finding instruments that support the standard, it can be a difficult and daunting process to find the relevant manufacturers and have confidence in the product.  Measurement Science LLC has decided to simplify this process, by developing www.SDI12.com (and www.SDI12sensors.com), with the goals of:

  • Providing 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
  • Gather up-to-date news on developments related to the SDI-12 standard
  • Providing a store of in-house and 3rd party products that use the SDI-12 standard, with a goal of identifying the best in class of each instrument

If you have news relating to the standard, we welcome your contribution

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