[Gllug] re: SCSI vs SATA

[Richard Jones]

On Thu, Jun 10, 2004 at 12:30:11PM +0100, Christian Smith wrote:
> Until you have to route all those parallel tracks so that they're all the
> same length. I'm no PCB designer, but I guess a small number of very high
> speed tracks are easier to lay out than loads of high speed tracks, hence
> the move to 16 bit buses in RDRAM and HyperTransport.
> 
> Against the costs of everyone routing PCBs correctly, the one time cost of
> designing more complicated serial drivers is probably low.

Yes, I used to do this sort of stuff.  Once you start hitting 100s of
MHz or GHz speed, all the traces need to be the same length, each
signal needs a separate return path, you need a huge ground plane, and
you need to add resistors to the end of each signal to stop
reflections.  The resistors were a particular problem I seem to
remember - we actually had to build the boards, and measure the
signals with an oscilloscope to find the right size of resistor.  This
was nearly 10 years ago, I don't know if things have improved since.

There is software which can do the layout semi-automatically but it's
very expensive, takes a long long time to run, and requires manual
intervention.  And it doesn't really do the specialist stuff anyway
(eg. we always used to lay out clock traces by hand before letting the
software have a go at the rest).

Rich.

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