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I built a simple shield/daughter card, with 3.6v 1w zener diodes and what I believe are the appropriate resistors (2.2K pullup (to 5v) on D-, 1M pulldown on D+, and 68 ohms between the D+- lines and the Arduino ports 2 & 4), but it didn't work. I enabled DEBUG_LEVEL=2, and I see a continuous stream of "ff:", which appears to mean that the bus is constantly being reset.

Vista reports "USB device not recognized"

I'm running Vista on a Dell XPS1730, with the Arduino 012 codebase/compiler.

Has anyone seen this behavior, or have any suggestions on how to fix this?

Here is part of the debug log:

00:Cá01:
02:
ff:
03:
23: c3 00 ff 07 00 cd db
02:
ff:
02:
ff:
(lots more 02:, ff:)
02:
1d: 80 06 00 01 00 00 40 00
20: 4b 12 01 10 01 00 00 00 08 11 77
ff:
02:
20: c3 c0 16 df 05 00 01 01 02 ca 9b
ff:
02:
20: 4b 00 01 3f 8f
ff:
02:
(lots more 02:, ff:)
ff:
02:
1d: 00 05 02 00 00 00 00 00
20: 4b 00 00
02:
(lots more 02:, ff:)
ff:
02:
1d: 80 06 00 01 00 00 40 00
20: 4b 12 01 10 01 00 00 00 08 11 77
ff:
02:
20: c3 c0 16 df 05 00 01 01 02 ca 9b
ff:
02:
20: 4b 00 01 3f 8f

2.2 kOhm may be too high. Did you check the voltage levels? Also make sure that your zener diodes are really low power types. You do use zeners, do you?

The problem wasn't the resistor, but the zener. Initially, all I could find at the local parts stores was 1 watt zeners, but eventually I found 500mW 1N747A. With this new zener, it worked.

Looking at the datasheets I see that the electrical characteristics differ between the 1W and 500mW zeners, which is very counter-intuitive - just like others had reported, and whose advice I ignored. My bad.

Thanks for your help.