Overview

Decagon MPS-2 (Manufacturers webpage)

This sensor communicates digitally via the SDI-12 protocol. http://www.sdi-12.org/
The cRIO in Panel 02 contains an NI-9403 high speed digital I/O module. The FPGA on the cRIO is programmed with a custom SDI-12 communications emulator. The module is capable of communicating with approximately 26 of the 32 digital buses simultaneously, this limit is aparently a hardware issue related to the impedance of the SDI-12 sensor network (basically the current draw to communicate with all those sensors is more than the module can provide). To combat this problem, the SDI-12 network is polled in two parts - all 5TM sensors, then all MPS-2 sensors.

The digital signal, when decoded, forms a simple ASCII text string which identifies the sensor address, the matric potential, and the temperature in degrees Celsius. An example output string is "B-352+25.6" - this shows sensor address "B", potential of "-352" kPa, and temperature "25.6" Celsius.

The MPS-2 terminates to a custom junction board designed by Aztera (http://www.aztera.com/). The board provides a many-to-one junction for power, ground, and signal buses which allows for 32 SDI-12 sensors to be connected to a single signal bus.

Calculation Notes

Wiring Diagram