
1-4
Introduction
1.3 Benets of Ultra2 SCSI
Ultra2 SCSI is an extension of the SPI-2 draft standard that allows faster
synchronous SCSI transfer rates and denes a new physical layer, LVD
SCSI. LVD SCSI provides an incremental evolution from SCSI-2 and
Ultra SCSI. When enabled, Ultra2 SCSI performs 40 mega transfers per
second, which results in approximately double the synchronous transfer
rates of Ultra SCSI. The SYM53C895 can perform 16 bit, Ultra2 SCSI
synchronous transfers as fast as 80 Mbytes/s. This advantage is most
noticeable in heavily loaded systems or large-block size applications
such as video on-demand and image processing.
One advantage of Ultra2 SCSI is that it signicantly improves SCSI
bandwidth while preserving existing hardware and software investments.
The primary software changes enable the chip to perform synchronous
negotiations for Ultra2 SCSI rates, and to enable the clock quadrupler.
Ultra2 SCSI uses the same connectors as Ultra SCSI, but can operate
with longer cables and more devices on the bus.
Chapter 2 contains
more information on migrating from an Ultra SCSI design to support
Ultra2 SCSI.
1.4 TolerANT Technology
The SYM53C895 features TolerANT technology, which includes active
negation on the SCSI drivers and input signal ltering on the SCSI
receivers. Active negation drives the SCSI Request, Acknowledge, Data,
and Parity signals active high rather than allowing them to be passively
pulled up by terminators. Active negation is enabled by setting bit 7 in
the STEST3 register.
TolerANT receiver technology improves data integrity in unreliable
cabling environments, where other devices would be subject to data
corruption. TolerANT receivers lter the SCSI bus signals to eliminate
unwanted transitions without the long signal delay associated with
RC-type input lters. This improved driver and receiver technology helps
eliminate double clocking of data, the single biggest reliability issue with
SCSI operations. TolerANT input signal ltering is a built in feature of the