In a message dated 03/01/2000 11:27:50 PM Eastern Standard Time,
[log in to unmask] writes:

<<  Hello,

  Can any one tell me why UDMA 66 requires 80wires when it only uses 40 pins?
  It just doesn't seam intuitive.
  Thx,
  Andrew >>

Hi,
  When a signal travels down a wire, it "induces" a signal in any wires
running next to that wire. This inductive coupling increases with frequency,
so the higher the frequency, the greater the induced signal will be in the
nearby wire(s). With the UDMA/66 specification being for 66Mhz, up from 33Mhz
with the older spec, the higher frequency caused the inductive coupling
between wires to become great enough to cause problems with signal integrity.
I.E. a logical "1" going down one wire could induce enough signal in a second
wire next to it to cause the second wire to also register a "1", even though
no signal was sent down the second wire.
  To get around this problem, they doubled the number of wires in the cable
(from 40 to 80), and grounded every other wire (even numbered wires carry
signals, odd numbered wires are grounded). Now when a signal goes down a
wire, the 2 wires (one on each side of the first wire) that would normally
have the "false" signal induced on them, are grounded. Any induced signal in
these wires is now shunted to ground, eliminating the problem.

HTH,
Peter Hogan
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