[Gllug] re: SCSI vs SATA

[Andrew Halliwell]

And verily, didst Richard Jones announce to the hordes:
> 
> 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.

Well, when you get into those frequencies, the tracks on a pcb stop being so
much tracks, as waveguides. Lots of redundant track on a board to keep the
tracks the correct length and stuff, iirc. All based on frequency to prevent
standing waves and interference.
 
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