[Al Skierkiewicz]

Seth,
That meter, as most modern meters, apply a current to the probes when in the "diode" mode and then measure the voltage drop directly. In a normal diode or transistor junction, the meter would read 0.7v. The specs say that the current in this meter is limited to 1 ma when in the diode mode.
I have to ask, was the "ON" indicator on the sensor illuminated and the output was not sensed by the roboRio? The RoboRio has a 40k pullup resistor to 3.3 volts which may not be enough to pas current through the Banner sensor.