Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!evax!utacfd!merch!cpe!adaptex!adaptx1!neese
From: neese@adaptx1.UUCP
Newsgroups: comp.periphs.scsi
Subject: Re: <13400@unixland.uucp>
Message-ID: <283400055@adaptx1>
Date: 26 Feb 91 18:27:40 GMT
References: <13400@unixland.uucp>
Lines: 25
Nf-ID: #R:unixland.uucp:13400:adaptx1:283400055:000:1122
Nf-From: adaptx1.UUCP!neese    Feb 25 12:06:00 1991


>/* ---------- "speed difference on external drives" ---------- */
>
>Does having a SCSI drive external to a 386 system, on a cable
>several feet long, affect the data transfer rate?  Is it significantly
>better to have the drive mounted internally in the system?

The length of the cable (i.e. SCSI bus) does affect the data transfer rate.
Although the effect is minimul.  For every foot of cable you cause a slew
in the SCSI bus timing of 2ns.  So with a little math:

If you have a 6 foot cable (includes the internal and external), and are
transferring a block's worth of data (UNIX) you would incur approximately
(6 * 2 * 1024) 12.288 micro-seconds of delay in the overall transfer for each
and every transfer.
And of course, the longer the cable and the bigger the transfer the longer the
overall delay gets.  This example does not take into account the selection,
command, and status phases that occur for each transfer as well.

			Roy Neese
			Adaptec Senior SCSI Applications Engineer
			UUCP @  neese@adaptex
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