HyQuest Solutions iRIS Under Cover, User Manual

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Appendices
8.2 SDI-12
This chapter contains the following subsection:
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What is SDI-12?
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Advantages of SDI-12
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SDI-12 Electrical Interface
8.2.1 What is SDI-12?
SDI-12 stands for S erial D igital I nterface at 12 00 bps. It is a standard to interface battery powered data recorders with
microprocessor-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 microprocessor makes the computations, converts sensor readings into the appropriate units, and uses the SDI-
12 protocol to transfer data to the recorder.
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.
8.2.2 Advantages of SDI-12
A serial-digital interface is a logical choice for interfacing microprocessor-based sensors with a data recorder. This has
advantages for sensors and data recorders.
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Unique and complex self-calibration algorithms can be done in microprocessor-based sensors.
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Sensors can be interchanged without reprogramming the data recorder with calibration or other information.
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Power is supplied to sensors through the interface.
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Hybrid circuit and surface mount technologies make it practical to include the power supply regulator, a
microprocessor, and other needed circuitry in small sensor packages.
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Sensors can use low cost EEPROMs (electrically erasable programmable read only memory) for calibration coefficients
and other information instead of internal trimming operations.
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The use of a standard serial interface eliminates significant complexity in the design of data recorders.
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Data recorders can be designed and produced independently of future sensor development.
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SDI-12 data recorders interface with a variety of sensors.
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SDI-12 sensors interface with a variety of data recorders.
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Personnel trained in SDI-12 will have skills to work with a variety of SDI-12 data recorders and SDI-12 sensors.
8.2.3 SDI-12 Electrical Interface
The SDI-12 electrical interface uses the SDI-12 bus to transmit serial data between SDI-12 data recorders and sensors. The
SDI-12 bus is the cable that connects multiple SDI-12 devices. This is a cable with three conductors:
1) A serial data line
2) A ground line
3) A 12-volt line
In the following specifications, all values not indicating specific limits have an allowable tolerance of ±10% of the value.
The SDI-12 bus is capable of having at least 10 sensors connected to it.
For more information, see the following subsections:
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Serial Data Line
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Ground Line
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Volt-Line
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