Parallel vs Serial speed

[rlk at alum.mit.edu (Robert L Krawitz)]

   Date: Wed, 07 Feb 2007 10:31:52 -0500
   From: Matthew Gillen <me at mattgillen.net>

   markw at mohawksoft.com wrote:
   > It has nothing to do with synchronization, if anything, serial is
   > a harder interface because it requires circuitry or software to
   > convert it to parallel data from serial data.

   I'm sure you're correct about all the voltage analysis, except that
   "harder" from the above must not equate to "cheaper", since you
   don't see any computers with Universal Parallel Bus ports...

Converting from parallel to serial isn't that hard if the data is
already parallel -- it's basically just a shift register.  The hard
part about traditional bit-parallel interfaces is that each bit has to
be kept in sync -- if you have a 32-bit wide interface, that means
that 32 bits have to be kept in sync at high clock frequencies so that
the data gets reassembled correctly at the other end.  Differences in
the length of each conductor can become important.  A serial
interface, on the other hand, merely has to preserve the ordering
between the bits (basically, not allow one wavefront to overtake
another) -- a much simpler task.

Keep in mind that serial cables are much simpler than parallel cables,
and crosstalk is much less of an issue, so it's easier to clock it up.

   /Something/ about parallel interfaces obviously makes them more
   expensive than serial for the same speed, and I didn't see anything
   in your post that explained /why/ "High speed drivers are
   expensive" compared to serial ones.

Newer interconnects (PCI Express, Infiniband) use a hybrid approach,
using scalable serial connections (so with PCI-E, you can have
anywhere between 1 and 16 lanes, with bandwidth scaling with the
number of lanes).  The lanes aren't clocked together and don't need to
be perfectly synchronized.

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