Appendix A - SDI-12 Communication
SDI-12 (serial data interface at 1200 baud) communications protocol allows compatible devices
to communicate with each other. More information about SDI-12 can be found at
SDI-12 Wiring Information
The SDI-12 Hydra Probe has three wires. A ground wire, a +12 volt power wire and a blue data
wire. The data wires from multiple probes can be connected in parallel to a terminal assembly. A
single data wire from the terminal assembly can then be connected to the SDI-12 data port on
the logger.
Power Requirements
9 to 20 VDC (12VDC Ideal)
Red Wire
+Volts Power Input
Black Wire
Ground
Blue Wire
SDI-12 Data Signal
Baud Rate
1200
Power Consumption
<1 mA Idle 30 mA Active
Table 2.2 Digital SDI-12 Hydra Probe II Information.
Addressing an SDI-12 Sensor
Data is communicated digitally between the probe and the logger on the single data wire.
Communications over this single wire is necessarily half duplex. That is, only one device can
send data at a time. SDI-12 communications is of the master/slave variety. The logger is the
master and all of the probes are slaves. The probes must each have a unique address. This must
necessarily be set before the probes are all connected. The probes reply with information only
when they have received a valid command that is addressed to them.
It is important to note that each SDI-12 sensor must have its own unique address. If two sensors
on a logger have the same address, it will disrupt communication between the loggers and the
probes. The default address for the Hydra Probe is “0”. The user must change the address of the
probe from 0 to another letter or digit as the probes are added to the terminal assembly. The data
logger's “Transparent Mode” can be used to set this address.
Changing SDI-12 Parameter Sets
In Transparent Mode, the parameter set can be changed to an alternate parameter set or set to any
parameter set the user wants. For the example below, the address of the probe is 1.
A)
Type “
1M!
” . the response should be
10029
. This means that a sensor with an
address of 1 will take 2 seconds to return 9 parameters with one or more D
commands.
B)
Type “
1XM!
”. The response should be
1HJFGOKMLN
.
This means that the sensor
with address 1 has been set to the first default Parameter set. The letters correspond to
parameters that the Hydra Probe is capable of measuring in this set. Most users are
only interested in H, J, F, and G. If the probe has another parameter set, you may
wish reset the probe to the first parameter set by typing “
1M!
”
or the user may type
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