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Input source does not read zero when plugged into digital input (but works fine on analog input)

I was using the analog inputs on the NI USB-6009 to measure the signal from a sensor. Everything worked fine.
Since the output from the sensor is basically just a pulse ("hey, I've been triggered"), I'm trying to use the digital inputs to free up the analog inputs. Again, the output is 0 volts when not triggered, and 5V when triggered. But when I connect it to a digital input and measure the voltage with a multimeter, it gives me somewhere between 1.5 and 3.5 volts when it's supposed to be 0. When the device triggers, it goes to 5V as expected. I imagine this is a voltage divider issue. I tried using an inverter to buffer the sensor output, but it didn't help.
Any ideas? Thanks.
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Nevermind, I got the inverter to work. Need to check my connections more carefully Smiley Sad
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