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VP-3
6 COMMUNICATION
6
Communication
The Decagon VP-3 sensor can communicate using two different meth-
ods, Decagon serial string or SDI-12 communication protocol. In this
chapter we briefly discuss the specifics of each of these communica-
tion methods. Please visit
www.decagon.com/support/literature
for
the complete Integrator's Guide, which gives more detailed explana-
tions and instructions.
6.1
Decagon Serial String
When excitation voltage is applied, the VP-3 sensor makes a mea-
surement. Within about 140 ms of excitation two measurement val-
ues transmit to the data logger as a serial stream of ASCII characters.
The serial out is 1,200 baud asynchronous with 8 data bits, no par-
ity, and one stop bit. The voltage levels are 0 to 3.6 V and the logic
levels are TTL (active low). The power must be removed and reap-
plied for a new set of values to be transmitted. The ASCII stream
contains two numbers separated by spaces. The first number is vapor
pressure in kPa with a resolution of 0.01 kPa and the second number
is temperature in C with a resolution of 0.1 C. A carriage return
follows the two numbers, then the character 'p,' indicating that this
is a CTD sensor, then a checksum character, and finally a carriage
return and line feed.
SDI-12 Communication
The VP-3 can also communicate using SDI-12 protocol, a three-wire
interface where all sensors are powered (white wire), grounded (bare
wire), and communicate (red wire) on shared nodes (for more infor-
mation on the SDI-12 protocol, visit www.sdi-12.org). Below is a
brief description of SDI-12 for communication. If you plan on using
SDI-12 for communication with the VP-3 Sensor, please see our Inte-
grator's Guide at
www.decagon.com/support/literature
for detailed
instructions.
SDI-12 Commands
15
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