Path: utzoo!attcan!uunet!decwrl!public!thad
From: thad@public.BTR.COM (Thaddeus P. Floryan)
Newsgroups: comp.periphs.scsi
Subject: Re: <13400@unixland.uucp>
Message-ID: <1942@public.BTR.COM>
Date: 27 Feb 91 11:56:47 GMT
References: <13400@unixland.uucp> <283400055@adaptx1>
Organization: BTR Public Access UNIX, MtnView CA, Contact: cs@btr.com 415-966-1429
Lines: 31

In article <283400055@adaptx1> neese@adaptx1.UUCP writes:
>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.

Uh, Roy, I beg to differ with your conclusions in this regards.

The delay, using your example, "should "be a CONSTANT 12nS upon the initiation
of data transfer, not an additive delay with each new byte (unless my
understanding of SCSI is really skewed, for which I'd appreciate clarification)

Given my (original) example 20 foot cable, I'd be seeing some 40 microSeconds
of delay on my system by your calculation and THAT'd be definitely noticeable
as it's my boot (aka "system") disk (a Quantum 80S) that's at the far end!  :-)

It is (was? :-) my understanding that once a transfer starts, all requested
bytes are placed on the bus and thus no additional delays would be seen after
the (in my case (20')) initial 40nS delay.

I (and, I'm sure, others) would appreciate your comments (and, if I'm wrong,
don't hesitate to flame! :-)

Thad Floryan [ thad@btr.com (OR) {decwrl, mips, fernwood}!btr!thad ]