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46 PCM-3420 User's Manual
F
FAST SCSI
Provides for performance and compatibility enhancements to
SCSI-1 by increasing the maximum synchronous data transfer rate
on the SCSI bus from 5 MBytes/sec to 10 MBytes/sec.
There are 2 handshaking modes on the SCSI bus, used for
transferring data: ASYNCHRONOUS and SYNCHRONOUS.
ASYNCHRONOUS is a classic Req/Ack handshake. SYNCHRO-
NOUS is “sort of” Req/Ack, only it allows you to issue multiple
Req’s before receiving Ack’s. What this means in practice is that
SYNCHRONOUS transfers are approx 3 times faster than ASYN-
CHRONOUS.
SCSI-1 allowed asynchronous transfers at up to 1.5 Mbytes/Sec
and synchronous transfers at up to 5.0 Mbytes/Sec.
SCSI-2 had some of the timing margins “shaved” in order that
faster handshaking could occur. The result is that asynchronous
transfers can run at up to 3.0 bytes/Sec and
synchronous transfers at up to 10.0 Mbytes/Sec. The term
“FAST” is generally applied to a SCSI device which can do
synchronous transfers at speeds in excess of 5.0 Mbytes/Sec.
This term can only be applied to SCSI-2 devices since SCSI-1
didn’t have the timing margins that allow for FAST transfers. A 10
MHz transfer rate for SCSI came out of a joint effort with the IPI
(Intelligent Peripheral Interface) committee in ASC X3T9.3. Fast
SCSI achieves 10 Megabytes/second on the A-Cable and with
wider data paths of 16- and 32-bits can rise to 20 Megabytes/
second and even 40 Megabytes/second. However, by the time the
market starts demanding 40 Megabytes/second it is likely that the
effort to serialize the physical interface for SCSI-3 will attract high-
performance SCSI users to the Fiber Channel.
A word of caution. At this time the fast parameters cannot be met
by the Single Ended electrical class, and is only suitable for
Differential. One of the goals in SCSI-3 is to identify the improve-
ments needed to achieve 10 MHz operation with Single Ended
components.