TDR SOIL MOISTURE MEASUREMENTS
3
3.1 ASSIGNING ADDRESSES
The SDM1502 and SDMX50 are Synchronous
Devices for Measurement (SDM); the
datalogger communicates with these devices
via control ports 1, 2, and 3 (and analog
channel 1H on the 21X). Addresses set in the
devices allow the datalogger to communicate
with the correct device.
There are sixteen possible addresses; a
maximum of four are used in a TDR system.
The address assigned the SDM1502
determines the addresses that need to be
assigned to the multiplexers: The multiplexer
with its input connected to the cable from the
1502B is level 1, level 2 multiplexers are those
connected to the level 1 multiplexer, level 3
multiplexers are those connected to a level 2
multiplexer. No more than 3 levels of
multiplexers are allowed. The level 1 address is
1 greater than that of the SDM1502. All level 2
multiplexers have the address set to that of the
SDM1502 plus 2 and all level 3 multiplexers
have the address set to that of the SDM1502
plus 3. The addresses are in base 4 (Table 3-
1); for example, if the SDM1502 address is 22,
the addresses for level one, level two, and level
three multiplexers are 23, 30 and 31,
respectively.
The address for the SDM1502 is set with
switches; the switches can be reached through
an access hole in the case (Figure 3-1). Table
3-1 lists the address settings.
FIGURE 3-1. Address Switch on SDM1502
The address for the SDMX50 is set with
jumpers. There are two jumpers on each
multiplexer. The jumpers are labeled MSD for
Most Significant Digit and LSD for Least
Significant Digit. Each jumper has four pairs of
pins. Depending which pair of pins is
connected with the jumper, the digit can have
the value of 0, 1, 2, or 3 (Figure 3-2). Table 3-1
lists the address settings.
FIGURE 3-2. Location of Address Jumpers on SDMX50
