VP-3
User’s Manual
5. Communication
18
5. Communication
The Decagon VP-3 sensor can communicate using two different
methods, Decagon serial string or SDI-12 communication
protocol. In this chapter we will briefly discuss the specifics of
each of these communication methods. Please visit
www.decagon.com/support/literature for the complete
integrator's guide, which gives more detailed explanations and
instructions.
Decagon Serial String
When excitation voltage is applied, the VP-3 sensor makes a
measurement. Within about 140 ms of excitation two
measurement values are transmitted to the data logger as a serial
stream of ASCII characters. The serial out is 1200 baud
asynchronous with 8 data bits, no parity, and one stop bit. The
voltage levels are 0-3.6V and the logic levels are TTL (active low).
The power must be removed and reapplied for a new set of
values to be transmitted. The ASCII stream contains 2 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.
