| Ultra2 SCSI Low
Voltage Differential (LVD) Drives
Seagate’s
high-performance Cheetah and Barracuda drives (drive models with a suffix of LC or LW - click here for list) are ushering in
the future by utilizing the super-fast LVD Ultra2 SCSI parallel interface capabilities.
Users of video, database servers, RAIDs, workstations and high-end desktop applications
benefit from the greater I/O bandwidth, device connectivity, data reliability, and cable
lengths offered by LVD. With the capability to multi-mode, Seagate LVD drives are backward
compatible with single-ended Narrow and Wide SCSI devices (not High Voltage
Differential) and provide cost savings, rapid bus rates and increased reliability.
Understanding the
Terms
Low Voltage Differential
(LVD) devices are defined under the Ultra2 subset of the SCSI-3 standard. Industry
wide, the terms LVD and Ultra2 SCSI are used interchangeably.
LVD, a
Logical Progression
LVD provides SCSI bus data
rates of 80 Mbytes/sec. That's double the fastest SCSI-2 standard (40 Mbytes/sec), and
light years ahead of the SCSI-1 standard used prior to 1992 in which SCSI bus rates were
as slow as 3 Mbytes/sec. This increased bandwidth means optimal performance for server
environments where rapid response is required and random access and large queues are the
norm. When using applications such as CAD and CAM, digital video and any RAID environment,
the increased bandwidth is immediately noticeable as information is moved quickly and
effortlessly.
LVD Benefits
LVD increases the maximum
burst transfer rates to 80 Mbytes/sec, provides differential data integrity, extends the
SCSI bus cable lengths to 25 meters (12 meters with 16 devices) and provides easy system
configuration for up to 16 devices. This is a dramatic increase from Ultra SCSI
single-ended cable restrictions of 3 meters and maximum burst transfer rates of 40
Mbytes/sec. The lower voltage requirements of LVD versus HVD allow for the integration of
the differential drivers and receivers into the drive's on-board SCSI controller. The
older Ultra HVD design requires separate and costly high-voltage components. As a result,
LVD provides the integrity of high voltage differential designs at the system cost
of single-ended I/O.
LVD
Compatibility
LVD is fully compatible
with the existing installed single-ended SCSI base. A unique DiffSens circuit determines
the type of SCSI bus the device is being used on, LVD or single-ended, and configures the
drive operation to the appropriate bus capability.
The following diagram
shows the three bus operational states and the resulting LVD compatibility. HVD devices
require a special controller and are not compatible with LVD or single-ended devices;
therefore, when DiffSens senses an HVD device, it disables the operation.
LVD is a subset of the
SCSI-3 standard. LVD devices will work on SCSI-1and SCSI-2 bus segments. Conversely, older
SCSI-1 and SCSI-2 single-ended devices will work on an LVD bus. Compatibility is an
important SCSI feature, but when using SCSI devices of different vintage on the same SCSI
bus, all peripherals on that bus will respond to the earliest version SCSI specification.
If an Ultra device is installed on an LVD bus, all devices on the bus will respond at
Ultra mode.
LVD
Specification
The 16-bit Wide SCSI
connection is required to achieve the maximum LVD bus data rates of 80 Mbytes/sec. Because
of this requirement, the future of the 8-bit Narrow SCSI bus is limited, and Seagate has
no plans to produce 8-bit Narrow LVD devices. All Seagate designs will incorporate LVD
using the standard 68-pin Wide SCSI connector or the 80-pin single connector (SCA-2) when multiple drives and easy integration are
required. The low voltage and current component of LVD allows the differential
transceivers to be implemented into the drive's on-board SCSI controller, eliminating the
need for separate and costly external high voltage differential components.
LVD Summary
Disc drive data rates
double about every three years. Drives, with data rates of 10 Mbytes/sec, work well
with an Ultra SCSI bandwidth of 40 Mbytes/sec. The Barracuda and Cheetah drives have
recorded sustained transfer rates as fast as 20 Mbytes/sec. Multiple drive applications
require the 80 Mbytes/sec bandwidth provided by LVD. LVD also provides for easy
integration of up to 16 devices on the SCSI bus using 12 meters of cable, the data
integrity of differential at the same cost as the older single-ended designs and the
parallel SCSI bus performance needed for Seagate’s high-performance Barracuda and
Cheetah drive families. |
