FAQ Topic

Does the SediMeter or LogDator support SDI-12?

The answer is no. Here is why:

SDI-12 is an old 1200 baud serial communication standard for environmental monitoring instruments, used between intelligent sensors and data loggers. It is used for environmental monitoring with very low data transfer requirements. SDI-12 uses an electrical layer that is non-standard within the electronics industry, and with voltage levels that are incompatible with many modern low-power components, requiring extra hardware. SDI-12 defines both the electrical layer and the communication protocol. Data are sent in 7-bit ASCII code with measurement values already calibrated and converted to physical units.

The LogDator was developed primarily as a controller for the SediMeter, thus on the sensor side. Furthermore, the LogDator is a logger, so there is no need for a separate logger. If real-time data is required the LogDator has an interface that is 8 times faster, allows longer cables, and more instruments per network. The electrical layer is RS-485, the industry standard for instrument networks, which makes it easy to find modern low-power components. It is superior in every respect to SDI-12 in our application. We use our own open RS-485 protodol.

Update 2009-05-07: The LogDator can now be used also as a network logger, recording data both from SediMeters and other smart sensors, including third party sensors that support the communication protocol.

As noted, the transfer rate in our protocol is 8 times higher than in SDI-12. Even so, the measurement interval in a network with many instruments is limited by the communication speed. However, we can increase the baud rate up to 115,200 baud in our instruments just by editing software, giving a transfer rate 100 times higher than with SDI-12. The downside of a higher rate is higher power demand, why for most cases 9600 baud is a good compromise. A lower baud rate also allows for longer cables (although 1200 m is possible even at 115 kbps).

Footnote: The official name of the RS-485 standard has changed first to EIA-485, and now to TIA-485, as it has become an international standard.