Vaisala WS425 - Manual (Page 52)

USER'S GUIDE____________________________________________________________________
52 __________________________________________________________________ M210361EN-E
NOTE With Vaisala SDI-12 cable, code WS425CAB SDI the lead colors are as
follows: data - whi/yel; ground - Blk; and 12-volts - Brn.
The SDI-12 bus is can have at least 10 sensors connected to it. The bus
topology is a parallel connection, where each of the 3 wires of different
sensors are connected in parallel.
Serial Data Line
The data line is a bi-directional, three-state, data transfer line. Table 13
below shows the logic and voltage levels for the transmission of serial
data for the SDI-12 standard. The data line uses negative logic.
Table 13 Logic and Voltage Level for Serial Data
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
Voltage Transitions
During normal operation, the data line voltage slew rate must not be
greater than 1.5 volts per microsecond.
Impedance
When an SDI-12 device has its transmitter on, its direct current (DC)
source resistance must be greater than 1000 ohms and less than 2000
ohms. When the transmitter of any SDI-12 device is off, or in a low-
power standby mode, the DC resistance to ground must be within 160 K
to 360 K ohms. If an SDI-12 sensor does not use the 12-volt line for
power, its data line resistance to ground while powered down must be
within 160 K to 360 K ohms.