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Message-ID: <19971024000229.47394@mi.uni-koeln.de>
Date: Fri, 24 Oct 1997 00:02:29 +0200
From: Stefan Esser <se@FreeBSD.ORG>
To: Joerg Wunsch <joerg_wunsch@uriah.heep.sax.de>
Cc: freebsd-hackers@FreeBSD.ORG, Stefan Esser <se@FreeBSD.ORG>
Subject: Re: 2.2.2-RELEASE '875 SCSI won't negotiage
References: <19971021081759.TH50130@uriah.heep.sax.de> <199710220009.TAA18051@nospam.hiwaay.net> <19971022080341.HR59190@uriah.heep.sax.de>
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In-Reply-To: <19971022080341.HR59190@uriah.heep.sax.de>; from J Wunsch on Wed, Oct 22, 1997 at 08:03:41AM +0200
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On 1997-10-22 08:03 +0200, J Wunsch <j@uriah.heep.sax.de> wrote:
> As dkelly@hiwaay.net wrote:
> 
> >  Went back and RTFM'ed my Asus SC875 manual and obsverved on page 11 
> > the default Synchronous Transfer Rate (MS/Sec) (sic) is 20. So is this MB/
> > sec or MHz?
> 
> MHz.  Together with the 16-bit bus, it makes a theoretical maximum of
> 40 MB/s (minus transaction overhead which is, as Stefan mentioned, not
> neglicible).

The theoretical maximum is 40,000,000 bytes per second,
or 38.1 * 1024*1024 bytes per second (== 38.1 MB/s).
This would still allow for some 600 transfers of 64KB
each.

Fast drives offer a command overhead of some 50us with
a fast controller. If the bus is to be saturated, and the
maximum transfer length is 64KB, then the overhead is at
least some 30ms per second, or 3%.

We get a maximum net data rate of some 36.9MB/s, under
those assumptions.

Regards, STefan