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Interfaces:
The Physical Interface
This
is the interface connection and cabling standard that the device.
There are basically three different SCSI interface groups with each
hosting a number of varieties. The main groups are Single-Ended
(SE), High Voltage Differential (HVD) and Low
Voltage Differential (LVD). LVD is also called Ultra2 SCSI (note
the difference in name to Ultra SCSI-2 - they are completely different).
Each of these three main groups use completely different cabling
and connection standards. The most popular standard used in the
manufacture of SCSI devices until recently was Single-Ended. These
are the SCSI peripherals most people think of as SCSI, with a range
of single-ended data transfer rates and connectors such as SCSI-1,
SCSI-2, Ultra SCSI, Wide SCSI, Ultra-Wide SCSI, and SCSI-3. Each
of these standards use the same conceptual electrical interface
but use differing qualities & types of cables and connectors
to offer varying performance. Basically, the higher the cost of
the controller, cabling, and connections, together with a higher
level Command Set, the higher
the data throughput that can be achieved.
For example, SCS1-2 Narrow uses an 8-bit cable arrangement running
at 10Mhz delivering a maximum data throughput of 10MB/sec. SCSI-2
Wide uses a 16-bit cabling arrangement running at 10Mhz delivering
a data throughput of 20Mhx. Any device capable of meeting these
physical standards has the right to call itself SCSI-2 Wide or Narrow.
Ultra-Wide SCSI-3 uses a 68-pin connection over 20MHz cable running
at 16-bits to give 40MB/sec throughput.
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Low
Density 50-pin:
Used in many of the external sub-systems, this is the most popular
type of SCSI connector on the market. It is designed for SCSI-1
and some SCSI-2 applications. It has a maximum data-transfer
rate of around 5-10MB/sec. This connection is also referred
to at times as the Centronics 50-pin. |
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High
Density 50-pin:
Used mainly in the workstation environment, SUN officially
made this the connector of choice for many of their new lines
of SPARCstation's when they were originally released. These
connectors are .050 on center,. half the pitch of the 50 Pin
Centronics connector, retaining the same general shape, but
using pins instead of contacts. The maximum data-transfer rate
for this type of connector is a theoretical 10MB/sec. This is
also the connection used primarily Adaptec 2940U, 1542, 1540
etc. cards. This connection is also referred to at times as
the Micro DB50 or Mini DB50. |
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High
Density 68-pin SCSI-2 or 3 Wide:
This connection offers twice the data path and increased data-transfer
potential. This connector is also .050 on center, half the pitch
of the 50 Pin Centronics connector. The maximum data-transfer
rate for this type of connector is 20MB/sec to 40MB/sec dependant
upon environment. This connection is also referred to at times
as the Micro DB68 or Mini DB68. |
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Ultra2
LVD SCSI 68-pin High Density:
This connection is identical in appearance to the HD68 connector
below but offers twice the data path over Ultra-Wide SCSI-3
increasing the maximum throughput to 80MB/sec. |
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All diagrams are not to scale |
Note:
All
data transfer rates may vary dramatically from implementation to
implementation with other variables added to the equation such as
device type, device performance, host system and performance, cabling
quality, connector quality, and the number of other devices sharing
the bus.
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SCSI
Term
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Bus
Width (bits)
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Speed
(MB/sec)
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SCSI-1
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8
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5
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Fast
SCSI
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8
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10
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Fast
Wide SCSI
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16
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20
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Ultra
SCSI
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8
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20
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Ultra-Wide
SCSI
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16
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40
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Ultra2
SCSI (LVD)
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8
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40
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Wide-Ultra2
SCSI
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16
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80
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Wide-Ultra3
SCSI
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16
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160
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Related
topics:
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