glee said:
If you're building your own system / working at the mobo level, you look
at the manual to see which headers are USB and which are Firewire
(assuming they are not already marked as such on the PCB). I have been
building computers for some years and this seems like a far-fetched
problem that would occur only if someone was lazy and did not check
which header was which.... in which case they should stay out of the
computer's innards!
When Firewire headers were first introduced, they avoided this
problem, by making the header something like a 1x9. Nothing
like a 2x5 at all. And then, no danger of hooking up the
wrong stuff. Later on, they changed the connector.
The computer industry try to make the connectors, so mis-mating
is less likely. The 2x5 audio header has the keying pin in a
different position than the 2x5 USB header. So again, it's not
an ideal situation, but at least someone half-awake would
notice the keying problem when comparing those 2x5 connectors.
Properly designed computer case cable assemblies, would have
blocking plugs in the keying pin location.
But when two connector types are made *identical* , that's
irresponsible engineering. If I was the engineer who
thought this up, I wouldn't be proud of myself.
Sure, the installer should be omniscient. And we can always
blame the person installing, if it ends in a "puff of smoke".
As an omniscient, you probably also know that a significant number
of computer cases, have actually been mis-wired on the front panel.
Do you ever buzz out the wiring on the computer you're building,
to verify it ? I have. The secret here, is companies that make
computer cases are "metal bangers". They have no electrical
engineers on staff, and they buy cable assemblies from a
third party, without checking them. If the installer doesn't
verify the wiring, the end user could end up blowing a
$1000 camcorder on the front Firewire port, on the first try.
*******
To give you an idea of the extraordinary lengths you have to
go to, to use electronics, take my current situation.
I have a multimeter, with an RS232 interface. Now, the problem
is, if I connect to the computer, there's a potential the
meter is now referenced to the ground of the computer. If
I plug the meter probes into 120VAC say, to measure
house voltage and have the computer record the readings, there
could be a "puff of smoke" inside my computer.
(I worry about stuff like this.)
I consulted the multimeter manual. Read the RS232 section.
No mention of "optically isolated RS232 port". Using
the model number, Googled the stupid thing. No mention
by any users, that the thing was optically isolated.
I had to trace down a schematic for the multimeter, and
note the presence of two small chips on RS232. And
then I knew, it was optically isolated, and safe
to connect the multimeter to anything I want, while
connected to the computer. So the path
between the meter and computer, is protected by
a LED and phototransistor, which breaks the dangerous
copper path by instead using light for communications.
(RadioShack 22-168A a.k.a. Metex M3650D RS-232 optoisolation)
http://img266.imageshack.us/img266/1869/optoisolate.gif
(Picture of typical 817 chip)
http://www.kwantlen.ca/science/phys...envi2307/lab10/ACtoDCad/PC817optoisolator.png
The thing is, this is a "feature", a safety feature,
and should be proudly documented in the user manual
(as a way to justify the purchase price of the meter).
*******
Between documentation and design, there's still a long
ways to go, to making good product. So somebody else
doesn't have to be omniscient.
"om nis cient - having complete or unlimited knowledge,
awareness, or understanding; perceiving all things"
Yeah, right. Be ready for that puff of smoke
Learn to run fast...
Paul
