In my experience, differential signaling is of little help for ESD problems because of the limited common mode voltage range of the receiver. ESD can generate a hundred volts across a small imperfection in the way a shield is connected and this shows up as a common mode voltage at the receiver.
Because small parasitics can have a large effect with ESD problems, it is difficult to give meaningful advice on the problem without having detailed knowledge of the design of the device. It is a shame that the USB spec (probably 1.1 here) is not tolerant of transients. It would have been really easy to include this in the specification originally and really easy to design to such a spec, especially for something slow such as a mouse. USB devices seem to regularly disappear even without ESD. Every now and then I have to reconnect a mouse or keyboard or other USB peripheral (on several different computers) because USB does not retry if no answer on the first one (may have been fixed in later USB specs, but I doubt it). I have clients that complain about this "feature" of USB. To prevent USB drops from ESD you generally need a very high quality cable and connector combination and the cable shield has to be closely referenced to the signal ground of the receiver to prevent common mode overload problems. I have never liked USB hard drives because of the problems I have experienced with USB. Firewire, in my experience, has been much more robust. USB also requires CPU cycles and can slow down if the CPU is busy doing other things. Firewire does not require intervention by the CPU which part of the reason that Firewire 400 is faster than USB "480." Doug On 1/5/10 2:07 PM, John Woodgate wrote: > In message <[email protected]>, dated Tue, 5 Jan 2010, Fred > Townsend <[email protected]> writes: > >> If the discharge is interrupting the signal then the signal low and >> the shield ground are being combined at some point. USB is a >> differential signal. Sounds like something is connected wrong at >> either the mouse end or the USB receiver chip end. > > Whatever voltage is on the shield is ALSO on the differential pair, as > a common-mode signal. The CMRR may well not be enough to cope, and the > permissible common-mode voltage may be exceeded anyway. -- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: [email protected] \ _ / ] \ _ / Website: http://www.dsmith.org - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
