Can someone explain this to me?

[WillHuang]

I was attempting to interface a RFID badge reader to a microcontroller in order to get the badge ID number. To do this, I had to connect the DATA0 and DATA1 lines of the badge reader to the input pin of the microcontroller. The DATA0 and DATA1 lines are normally held high and they go low to indicate a binary 0 (DATA0) or 1 (DATA1).

Now, if I were to connect the two lines directly to the microcontroller, my readings would be inaccurate 40% of the time. However, if I were to stick a resistor inbetween the input pin and DATA lines, my readings are accurate 100% of the time. I don't quite understand what's going on here. I've googled around a little bit, and the most I could get out of was 'reflection' on the transmission line - which sorts of make sense to me.

Was hoping someone here would know more and could explain it a little more too.

Thanks!

[ElektroQuark]

Quote from: WillHuang on Today at 04:08:31

The DATA0 and DATA1 lines are normally held high and they go low to indicate a binary 0 (DATA0) or 1 (DATA1).

By whom? The reader? Or are some pullup resistors needed?

[rrinker]

 Sure sounds like the latter. Would explain the odd results - they go low just fine, but are expected to have pullup resistors to be pulled high, otherwise they just float and you get unexpected results.