2.1.1 Addressing & Programming
The digital versions of the Hydra Probe (the SDI-12 and RS-485 versions) can be connected in
parallel so that multiple probes can be connected to a single communications port of a data
logger or other device. When multiple probes are connected this way, each probe must be
assigned a unique address
before they are installed
. The methods used for both probes are
similar, but unique. In addition, the user can select which processing method for the probe to use
and select which data is to be transmitted.
2.1.2 Daisy Chaining Versus Home Run Wiring
If you are contemplating the installation of multiple probes over a large area, consideration
should be given to the physical layout of the cables. Digital probes can be connected in a “Daisy
Chain” manner, where each probe is spliced onto the cable of the previous probe. This can
reduce the amount of cable required along with the corresponding cost. However, this means that
splices with have to made, and will likely need to be done in the field. Further if a cable breaks
or a splice fails, all probes beyond that point will be out of service until the break is repaired.
“Home Run” wiring means that each probe has a dedicated cable that runs all the way back to the
data collection station. The advantages here are just the reverse of “Daisy Chaining”. If there is a
break in the cable, only that probe is affected. There are no splices to fail. The disadvantage is
that the cable requirements and associated costs will be higher.
2.1.3 SDI-12 Hydra Probe II
The SDI-12 version is digital and can be used with Data Loggers that support this
communications method. SDI-12 stands for Serial Data Interface at 1200 baud. SDI-12 was
developed in cooperation with the USGS (U.S. Geological Survey) and is a standard
communication protocol for environmental sensors and data loggers.
2.1.3.1
Transparent Mode
Transparent Mode allows the user to communicate directly with the Hydra Probe. This is
necessary in order to assign an address to the probe or modify the probe's configuration.
To program an SDI-12 Version of the Hydra Probe, an SDI-12 compatible device that supports
Transparent Mode is required. Most SDI-12 data loggers support Transparent Mode. The SDI-12
protocol is not compatible with common serial data communications, so a device is needed to
convert between the two. A typical method is to connect a Personal Computer (PC) to a data
logger using a standard nine pin serial data communications cable, and then the probe is
connected to the SDI-12 port on the logger, and power is supplied. A terminal program (like
Hyper Terminal) is started on the PC. Typically the user must issue a command to the logger to
enter Transparent Mode.
See Appendix A for specific information on SDI-12 commands for the Hydra Probe. Please visit
for more information about the SDI-12 Protocol. Table 2.2 describes the
physical specifications, wire designations, and other information about the digital SDI-12 Hydra
Probe II.
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