On Sat, 16 Mar 2002, Andrew Basterfield wrote:

> I was a little unclear, I meant in the context of a full modern 160Mb/sec
> system. The 160Mb/sec SCSI doesn't run at 160MHz because it is wider than
> the 8 bits of the old sparcs - I think 160Mb/sec is ultra-wide ie. 32bits,
> so it would only need to run at a sensible 40MHz. Could you imagine trying
> to get reasonable propogation characteristics at 160Mhz from unscreened
> ribbon cable? If you want to go faster than this there are (mainframe)
> devices that talk SCSI-over-FDDI but then you're back to a serial data
> stream.
> 
> NetBSD dmesg reports my SCSI (on my SS2) to be clocked at 5Mhz wheras
> OpenBSD reports it clocked at 7 point something MHz. It should be SCSI-2
> (fast) == 10Mhz. Am I missing something?

Hmm, I (conveniently?) forgot that the 160Mb/sec was UW.... but is it
actually 160MB/sec, i.e.MegaByte/sec as opposed to Megabit/sec
(Mb/sec). I don't know!

As to shoving data down ribbon cable at high rates, it is (has been)  
done at clock rates >> 100MHz. The systems treat the ribbon cable just
as a transmission line, and the PCB tracks just the same. With
computers using databus's up to 400MHz and faster, the PCB designer is
having to learn from the stripline designers song-sheet!.

Geoff
-- 
    Geoff Blake	              Please reply to:		Using Linux on  
    Chelmsford		geoff (a) palaemon . co . uk	Intel & NetBSD 
    Essex UK		  geoff @ g8gnz . ampr.org	 on Sun Sparc

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