14
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Network Cabling
Before connecting on a network each WET150 sensor must be
individually set up with a unique address. So they must be
individually connected to either to a GP2 logger controller, or
WET150 Meter or other SDI-12 controller.
This is also a good opportunity to change individual sensor
parameters, e.g. for soil type
Maximum cable length and network size
The SDI-12 standard requires that a cabled sensor network with a logger
must be capable of supporting at least 10 sensors, each with 200 feet (61m)
of cable in a star configuration back to the logger. It also states that longer
cable lengths are possible if the network contains fewer sensors.
However, there is no defined maximum individual or total cable length as
this depends on many factors including the numbers of sensors, network
configuration, supply voltage, cable type, logger type and environmental
factors (such as whether the cable is buried) and proximity to electrical
noise sources like transmitters and power transformers.
While it is not possible to test every network configuration, tests have been
carried out on various configurations with different cable lengths and
numbers of WET150s. One such scenario was fifteen WET150 sensors
daisy-chained 5m apart using extension cables and T-piece connectors, all
on a single 245m backbone (so 320 m in total).
The theoretical maximum number of sensors on any SDI-12 network is 62,
as defined by valid address ranges 0-9, A-Z, a-z. Tests so far have shown
that at least 35 WET150 sensors can be connected to one GP2
logger/controller. The test was performed using a star configuration with
each WET150 connected using 2m cables.
If the SDI-12 network also contains devices other than the WET150, a
reduced maximum number of sensors or reduced overall cable length may
be needed due to their potentially higher network loading
NOTE
