Is it possible to passively listen on low speed USB communication?
Suppose I connect a logic analyzer to D+ or D- line, will I see a valid data?
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I'm not certain about this, but I believe that if you look at the differential voltage between the two lines, you will see data, but it will be NRZI encoded. This means that the bits are encoded by whether the bus level changes or not. Change (in either direction) means 0 and no change (for one bit-time) means 1.
I believe there are also CRCs and other signaling bytes at the beginning and end of packets.
This is what you need http://www.totalphase.com/products/beagle_usb480/, it is a USB 2.0 protocol analyzer, comes with software that will parse out USB traffic, and will be much easier to use than a logic analyzer. I have one of these for I2C/SPI and it works well, plus, Bunnie uses it http://andrew.huang.usesthis.com/