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From: taylor@chris.Solbourne.COM (Dick Taylor)
Newsgroups: comp.periphs.scsi
Subject: Re: SCSI disk transfer modes.....
Message-ID: <1990Dec5.182853.25111@Solbourne.COM>
Date: 5 Dec 90 18:28:53 GMT
References: <28543@usc>
Sender: news@Solbourne.COM
Organization: Solbourne Computer, Inc.
Lines: 41

In article <28543@usc> rpinder@phad.hsc.usc.edu (Rich Pinder) writes:
>Some of the higher capacity SCSI disk drives rate transfer speed in both
>Synchronous as well as Asynchronous, with the former being rated as much
>faster.  Is synchronous transfer possible on an Intel based box??  I'm
>wondering if a NCR 486 microchannel, with the NCR 53C700 SCSI chipped
>controller, running SCO Unix could address these 5.0 MB/second speeds they
>claim?  Is it true that a limitation to Synchronous transfer is that there
>can only be one device running synchronous in a system??  

The transfer rates they quote are, of course, referring only to their
speed on the SCSI bus, and not to the throughput available from the drive.
The limit to throughput is still the media data rate, and there aren't
any drives out there with prolonged throughput anywhere near 5.0 MB/s.
(Or rather, there aren't any single-spindle readily-available
reasonably-inexpensive drives at that rate; there are a number of
leading-edge products with sustained transfer rates in excess of
5.0 MB/s, which is one of the reasons for the push to faster and
wider SCSI buses with SCSI-2.)

The advantage of synchronous transfer is that the actual bus occupancy
by any one device is much lower (the data is buffered, and intelligent
devices will disconnect and clear the bus while they don't need to
use it).  This allows you to put more devices on the bus without
contention problems.

The disadvantage of synchronous transfer is that both the host and
the device have to support it.  Synchronous support is routine for
modern high-capacity disk drives.  It's not always routine for
hosts, but a 53C700-based design certainly has the hardware to
support synchronous transfers.  As for the software, I happily
refer you to your vendor's support department.

>
>		Rich Pinder
>		USC School of Medicine
>		(213) 224-7099
>
>		rpinder@phad.hsc.usc.edu

			Dick Taylor
			Solbourne Computer, Inc.