Software-based analyzers [...] could someone explain this in more detail?
If you have the device working in Linux, then you can capture USB as it runs via Wireshark or other tools.
If you have the device working in Windows, I've used 'usbsnoop' and I think may be are other newer free software sniffers.
If the device is plugged into proprietary hardware or a locked-down computer (ie Kinect, or I have a Telstra T-Hub on my desk with a USB DECT transceiver soldered to its main board), then you need a hardware analyser. There may be other scenarios where software capturing doesn't work too well either, I've only done a tiny amount of it.
Wouldn't examining the USB packets on a computer, give you the same information?
AFAIK, even with a hardware sniffer you still do most of the actual analysis on the computer. The hardware device is just to sniff the raw data, and it normally has a second USB port to send it to your computer for analysis.
Before the Kinect hack launched countless discussion threads into a breakdown of USB sniffing and the like, I'm sure this project would never even have hit its goal.
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They say "Software-based analyzers are available, but only useful in certain limited applications.", could someone explain this in more detail?
Wouldn't examining the USB packets on a computer, give you the same information?
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Slightly off-topic, but if you want to analyse signals like RS232, SPI etc. this is really nice:
http://www.seeedstudio.com/depot/open-workbench-logic-sniffe...