232C and USB
are both bit serial, but USB has higher speeds,
friendly voltage levels, power from a host included and well-defined
protocol.
I question friendly voltage levels. 3.3 volt powered logic and chips
came just after them if I recall correctly.
I'm not sure what's supposed to be unfriendly about RS-232C. The major
unfriendly part there, it seems to me, is the space in the middle, -3V
to +3V IIRC, for which the logical signal state is undefined.
Though I suppose it's a question of "friendly to what/whom"....
The part of RS-232C I _really like_ is the impedance spec, the part
that says that any pins or combinations of pins may be shorted to one
another or to any voltage source within, I think, the valid RS-232C
voltage range (-15V to +15V or some such - I forget the exact limits),
with the shorts maintained indefinitely, without damage.
I also like the splitting of protective ground and signal ground.
You'd almost think it had actually been designed by competent
electronics engineers or somethin'....
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