Avago Technologies LSI53C140 - Manual (Page 21)

Interface Signal Descriptions 2-3
Ver. 2.1 Copyright © 1998–2001 by LSI Logic Corporation. All rights reserved.
2.1.1.1 LSI53C140 Requirements for Synchronous Negotiation
The LSI53C140 builds a table of information regarding devices on the
bus in on-chip RAM. The Synchronous Data Transfer Request (SDTR)
and Wide Data Transfer Request (WDTR) information for each device is
taken from the MSG bytes during negotiation. For all devices in the
configuration to communicate accurately with each other through the
LSI53C140 at Ultra2 (Fast-40) rates, it is necessary for a complete
synchronous negotiation to take place between the initiator and target(s)
prior to any data transfer. On a 16-bit bus, the LSI53C140 at Ultra2
approaches rates of 80 Mbytes/s. The LSI53C140 defaults to Fast-20
rates when a valid negotiation between the initiator and target has not
occurred.
2.1.1.2 TolerANT Technology
In the SE mode, the LSI53C140 features TolerANT technology, which
includes active negation on the SCSI drivers and input signal filtering on
the SCSI receivers. Active negation causes the SCSI Request,
Acknowledge, Data, and Parity signals to be actively driven HIGH rather
than passively pulled up by terminators.
TolerANT receiver technology improves data integrity in unreliable
cabling environments, where other devices would be subject to data
corruption. TolerANT receivers filter the SCSI bus signals to eliminate
unwanted transitions without the long signal delays associated with
RC-type input filters. This improved driver and receiver technology helps
eliminate double clocking of data, which is the single biggest reliability
issue with SCSI operations.
The benefits of TolerANT technology include increased immunity to noise
on the deasserting signal edge, better performance due to balanced duty
cycles, and improved SCSI transfer rates. In addition, TolerANT SCSI
devices prevent glitches on the SCSI bus at power-up or power-down, so
other devices on the bus are also protected from data corruption.