Welcome to the SDI-12 web-site. It is presented by the SDI-12
Support Group to inform you about technical specifications as well
as the group's activities.
SDI-12 stands for serial data interface at 1200 baud. It is a standard
to interface battery powered data recorders with micro-processor based
sensors designed for environmental data acquisition (EDA).
EDA is accomplished by means of a sensor, or sensors, and a data recorder
which collects and saves the data. SDI-12 is a standard communications
protocol which provides a means to transfer measurements taken by an intelligent
sensor to a data recorder. An intelligent sensor typically takes a measurement,
makes computations based on the raw sensor reading, and outputs the measured
data in engineering units. For example, an SDI-12 pressure sensor may take
a series of pressure measurements, average them, and then output pressure
in psi, inches of mercury, bars, millibars, or torrs. The sensor's micro-processor
makes the computations, converts sensor readings into the appropriate units,
and uses the SDI-12 protocol to transfer data to the recorder.
What requirements make SDI-12 particularly useful?
SDI-12 is used in applications with the following requirements:
battery powered operation with minimal current drain
use of one data recorder with multiple sensors on a single cable
use with microprocessor-based sensors that perform complex calibration
algorithms or make internal computations
These requirements are necessary to acquire hydrologic data at remote sites.
Most sites use battery powered data recorders to operate for long and unattended
periods of time.
Why use micro-processor based sensors anyway?
A micro-processor in the sensor may calibrate the sensor, control sensor
measurements, and convert raw sensor readings into engineering units. The
micro-processor also controls the SDI-12 interface. It accepts and decodes
instructions received from the data recorder, starts the measurements,
controls all timing, and uses the SDI-12 protocol to communicate with the
data recorder.
Can more than one parameter be measured with a single data recorder?
Yes. SDI-12 is a multi-drop interface that can communicate with multi-parameter
sensors. Multi-drop means that more than one SDI-12 sensor can be connected
to a data recorder. The SDI-12 bus is capable of having ten sensors connected
to it. Having more than ten sensors, however, is possible. Some SDI-12
users connect more than ten sensors to a single data recorder.
Multi-parameter means that a single sensor may return more than one
measurement. For example, some water quality sensors return temperature,
conductivity, dissolved oxygen, pH, turbidity, and depth.
SDI-12 is used in water resource research and management, industry, government,
and agriculture. For example, the U.S. Geological Survey uses over 4000
SDI-12 sensors in its data collection networks. SDI-12 sensors are available
that measure the following:
