> Sorry if I am repeating a point somebody already made. I seem to remember
> from getting training on the U66, that the whole point of having double
> wiring is to be able to use half the wave length, gaining speed. By
> inverting the signal in one of the cables, the combined amplitude
> can remain
> the same but run at double speed. Simple, but reasonably clever.
You've got that wrong. What you are describing is differential
signalling, where the same signal is sent down a pair of conductors,
but is inverted in one of them. The receiver then looks at the
voltage _difference_ between the signals, rather than the absolute
voltage _levels_. This means that an common-mode (equal on both
signals) interference gets cancelled out:
For example, the signals A and -A are transmitted. Noise or
interference causes the received signals to be (A+N) and (-A+N).
Subtracting these signals at the receiver gives
(A+N) - (-A+N) = A + N + A - N = 2A
This therefore improves noise immunity. This doesn't inherently
give you any speed improvement. However, the improved noise immunity
allows you to use smaller signals and faster slew rates (how fast the
signal level changes), and this allows you to use faster bit rates.
So the question comes down to "are the extra conductors in a UDMA66
cable used for differential signalling or for ground wires"?
Later,
Kenn
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