USB cautionary tales
louie at transsys.com
Fri Jun 15 09:36:48 CDT 2012
Back in the late '80s or so when I was working at the compute center at U of MD in
College Park, we had a department that ran 10Mbit Ethernet coax between two building.
As Mike described, there was a lightning strike and a bunch of Ethernet transceivers
of the "vampire tap" variety got blown up. They were, of course, conveniently
located in the suspended ceiling over a couple of floors.
We got involved and looked around. Noticed a bit more than 10V difference in the
ground potential between the two buildings in question, as I recall. And the shield
on an RG-8/-213 equiv. coax cable can carry a lot of current when it was tied to
the building ground on either end!
On Jun 15, 2012, at 10:28 AM, Mike O'Dell wrote:
> This is a pretty good intro-level article.
> The pix of the crispy critters at the end of the article are worth the price of admission.
> This is germane to Hams because some of them (nobody around here, of course - big grin)
> run yer HIGH POWER, especially on the Squawk Radio long waves. If some of that RF got loose
> in the shack (radiating coax shield, high SWR, etc), USB cables could look mighty inviting.
> This also applies to Ethernet, although not quite so bad. In the early days of Ethernet
> at Xerox PARC, when they were using CATV coax and cable taps, the shield of the
> cable was tied to building ground. This was no problem at first. Then they extended
> the cable into another building and followed the same rules. OOPS!
> The local grounds in Building A and Building B were now tied together through
> the coax shield. When Building A took a very-near miss lightning strike, all the
> equipment on the Ethernet in Building B exploded before the cable had time
> to burn open at the entrance to Building B. That harsh lesson on the destructive power
> of "ground bounce" was taken to heart and is why the original DIX Ethernet 10-megabit
> standard required transformer-coupling, and the isolation requirement remains the case to this day.
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