john1993:
Docedison:
The first is a bad copy... It 'should' work but I'd not use it W/O the diodes.. It's very likely to be 'intermittent' when the diodes that are missing aren't doing their job.Designing electronics equipment isn't about how many parts you can throw into a circuit. It's about understanding how it is supposed to work and how it must interface with 'other' stuff. Arbitrarily chopping parts out because you don't Think... that they are required would Indicate that you know more than the original designer... were that truly the case...
YOU WOULDN'T BE ASKING WHAT AND WHY HERE... IMO
Doci dont have a doctorate (yet), only ee. but one of the first things ive learned is to design according to how things ACTUALLY work and not how they SHOULD or are SUPPOSED to work. i dont mean throwing something together and if it works ship it. but there are usually reasons for paticular components.
checking some app notes for northbridge chips i found the zeners ARE requred. not so much for protecting from chassis grounds or test equipment though but "ringing" and in this case clamping 5v signals from the mega8. since its running directly off the usb vcc there is potential to violate max Vih for the motherboard device. this can result in shutdown which i think is the reason my simplified version failed but the ebay unit works w/o diodes. the ebay mega8 is powered off a 3.3v ldo so didnt trigger an exception.
to test this i went back and altered my kluge to run off 3.3v. guess what... it worked. there are other reasons for the zeners and 68ohm involving slew rates and setup times but my circuit was close enough for usb1.0 operation.
people are of course free to add as many caps, resistors, diodes, or whatever. but personally i choose do so for a reason, not because i think they are SUPPOSED TO or SHOULD be there. ymmv.
What college did you get your EE degree from?
