When confronted by confused designers, I explain the USB feature I call the "common-mode noise detector". USB detects a new device connection via a very simple non-differential circuit, even though the communication is otherwise differential.
This problem was made noticable by the insistance of the designer to abide by the original USB specification of a floated capacitor-coupled shield on the peripheral connector, spoiling the effectiveness of an otherwise suitable shielded/screened cable. Anyway, this circuit happily responds to common mode transients, such as EFT/ESD and the like. What happens next is the software, responding to a "new device" signal, becomes confused when it finds a peripheral ALREADY enumerated (logged into the driver) and doesn't know what to do (really due to poor stress-testing of the software). Some of the first USB drivers of Window95 would do very entertaining things, drop into random sections of the communication protocol, or the infamous blue screen of death. If your test system is still running Windows95, you will have no chance to pass. Microsoft stopped supporting that driver. I found that Windows98 was much better, but not foolproof. USB (and Firewire) unsuitable as they are, have crept into industrial applications that the designers considered primarily from a software functionality point of view. Eric Lifsey ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
