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iRIS 220 / iRIS 320, V1.19 User Guide - 66
66
iQuest (NZ) Ltd - PO Box 15169, Hamilton, New Zealand Tel: +64 7 857-0810 Fax: +64 7 857-0811 Email: iquest@iquest.co.nz
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.
Serial Data Line
The data line is a bi-directional, three-state, data transfer line. Table 1 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 1. Logic and voltage levels for serial data
Voltage Transitions
During normal operation, the data line voltage slew rate must not be greater than 1.5 volts per microsecond.
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.
12 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 – September 17, 2002
Prepared By
SDI-12 Support Group
(Technical Committee)