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Appendices
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.
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8.1.3.1
Serial Data Line
The data line is a bi-directional, three-state data transfer line. Table 19 shows the logic and voltage levels for the
transmission of serial data for the SDI-12 standard. The data line uses negative logic.
Condition
Binary state
Voltage range
Marking
1
-0.5 to 1.0 volts
Spacing
0
3.5 to 5.5 volts
Transition
undefined
1.0 to 3.5 volts
Table 21 – Logic and voltage levels for serial data on SDI-12 bus
Voltage Transitions
During normal operation, the data line voltage slew rate must not be greater than 1.5 volts per microsecond.
8.1.3.2
Ground Line
The ground line must be connected to the circuit ground and the earth ground at the data recorder. The sensor circuit
ground also must be connected to the ground line, but not normally to its own earth ground. If it is necessary to connect
the sensor circuitry to earth ground, a heavy (12 AWG or larger) ground wire should be connected between the sensor
earth ground and the data recorder earth ground for lightning protection.
The ground conductor should be large enough to keep the voltage drop between the data recorder and all sensors less
than 0.5 volts during the maximum combined sensor current drain.
8.1.3.3
Volt-Line
The data recorder (or the external power supply) provides between 9.6 volts and 16 volts to the 12-volt line, with respect
to ground, as measured under a maximum sensor load of 0.5 amperes. SDI-12 does not require the data recorder to be
the source of power to the 12-volt line. Sensors connected to the 12-volt line must not have inductive loads. SDI-12 does
not require voltage limiting for transient protection in the sensor. Transient protection is however recommended.
Note
:
This information is taken from:
SDI-12 Serial-Digital Interface Standard for Microprocessor-Based Sensors,
Version 1.3 – January 28, 2016
Prepared By
SDI-12 Support Group
(Technical Committee)
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