On Thu, Apr 13, 2000 at 09:56:19PM +0100, Paul Jakma wrote:
> On Thu, 13 Apr 2000, John P. Looney wrote:
>> The extra wires are just grounding wires. As the bus runs at a higher
> frequency, there is more crosstalk/interferenece between data cables, so
> wrapping an extra ground wire around each one reduces that a lot (like the
> way twisted pair works).
>> twisting a ground wire around a signal wire is next to useless. (in
> fact it might make things worse because of inductance).
I haven't seen one of these cables yet, but if they are just high-density
ribbon cable, then the ground wires run in parallel with the signal wires
with no twists. In this configuration, the ground wires reduce crosstalk
by reducing the capacitance between adjacent signal conductors, at the
expense of increasing capacitance between signal and ground (which can
be compensated for, to some degree).
It would probably make the cable act more like a transmission line (or
rather a bunch of transmission lines), which makes it more predictable
and, thus, easier to design for at high frequencies.
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