5
CONNECTING TO A LOGGER
MPS-2 & MPS-6
5.3
Using Non-Decagon Loggers
The MPS-2 and MPS-6 sensors communicate using two different
methods, Serial (TTL) and SDI-12. This chapter discusses the specifics
of each of these communication methods. Please see the complete
Integrator’s guide for more detailed explanations and instructions.
5.4
Communication
Serial Communication
When you apply excitation voltage, the sensor makes a measure-
ment. Within about 40 ms of excitation, two measurement values
transmit 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 to 3.6 V and the logic levels
are TTL (active low). The power must be removed and reapplied to
transmit a new set of values.
The ASCII stream contains two numbers separated by spaces. The
stream terminates with the carriage return character. The first num-
ber output is water potential in kilopascals. The second number is
the temperature in degrees Celsius.
SDI-12 Communication
The sensor can also communicate using the SDI-12 protocol, a three-
wire interface where all sensors are powered (white wire), grounded
(bare wire), and communicate (red wire) on shared wires (for more
info, go to
http://www.sdi-12.org
). Below is a brief description of
SDI-12 for communication. If you plan on using SDI-12 for com-
munication with the sensor, please see our Integrator’s guide un-
der the support tab on the MPS-2 and MPS-6 product pages at
http://www.decagon.com
.
There are several benefits and drawbacks regarding the SDI-12 pro-
tocol. One benefit is that you can connect up to 62 sensors to the
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