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SCSI (Small Computer System Interface)
The advantage to SCSI (pronounced
"scuzzy") hard drives is that the new Ultra-2 controller 16-bit bus can transfer
data at up to 80 megabytes per second (MBPS) and handle seven SCSI devices, allowing much
more room for future expansion. The ultra wide adapter accepts fourteen devices. One
circuit board or card to accommodate all the peripherals to be connected to the SCSI port
in a daisy-chain fashion, rather than having a separate card for each device, making
it an ideal interface for use with portable and notebook computers. Because of this
scalability the interface is widely used in servers. Scanners,
tape drives, cartridge drives and
CD-ROMs also use the SCSI interface. SCSI
gives you added flexibility, but comes at an additional cost most shoppers are unwilling
to pay for.
SCSI is a set of evolving ANSI
standard electronic interfaces developed at Apple Computer and still used in the
Macintosh, the present set of SCSIs are parallel interfaces. SCSI ports are built into
most personal computers today and supported by all major operating systems.
The original SCSI, now known as SCSI-1, evolved into SCSI-2 as it became
widely supported. SCSI-3 consists of a set of primary commands and additional specialized
command sets to meet the needs of specific device types. The collection of SCSI-3 command
sets is used not only for the SCSI-3 parallel interface but for additional parallel and
serial protocols, including Fiber Channel, Serial Bus Protocol (used with the IEEE 1394
Firewire physical protocol), and the Serial Storage Protocol (SSP).
Ultra160 SCSI is a natural progression in the continually
evolving and improving SCSI technology. In this latest breakthrough,
Ultra160 SCSI boosts the throughput of Ultra2 SCSI (80 MByte/sec
maximum) by sending two bits of data per clock cycle instead of one,
producing a maximum data transfer rate of 160 MByte/sec.
This approach, known as double-transition clocking, allows both
the data and clock lines to run at 40 MHz. Double-transition
clocking is only one of several advanced features of Ultra160 SCSI
that improves the existing design. The result is an interface that
can handle the demands of the market's most advanced hard drives.
Currently existing SCSI standards are summarized in the table below.
Technology
Name
Maximum Cable
Length (meters)
Maximum
Speed
(MBPS)
Maximum
Number of
Devices
SCSI-1
6
5
8
SCSI-2
6
5-10
8 or
16
Fast SCSI-2
3
10-20
8
Wide SCSI-2
3
20
16
Fast Wide SCSI-2
3
20
16
Ultra SCSI-2, 8-bit
1.5
20
8
Ultra SCSI-2, 16-bit
1.5
40
16
Ultra-2 SCSI
12
40
8
Wide Ultra-2 SCSI
12
80
16
Ultra-3 or Ultra-160
12
160
16
Types of Internal SCSI Connectors
IDC50 Male
Used for SCSI-1 / SCSI-2
IDC50 Male
IDC50 Female
Used for SCSI-1 / SCSI-2
IDC50 Female
Used for SCSI-3 / Ultra-2 /
Wide Ultra-2
68-pin Male
Types of
External SCSI Connectors
Others by Apple, IBM and SPARC not shown
8-bit SCSI (Apple/Mac)
Used for parallel, serial or scsi applications: modem,
null modem, laplink, printer, scanner, removable storage
drive, Apple SCSI.
25-Pin
DB25 Male
8-bit SCSI (Up to 5MB/sec) Used in older scanners,
controllers, external scsi device cases.
Centronics 50 Female
Proprietary SCSI-2 interface (rare).
Micro Centronics 50 Male
8-bit SCSI 2 (Up to 10MB/sec). Also known as HD
(High-Density) 50 & HP (Half-Pitch) DB50.
Micro DB50 Male
16-bit Wide, Wide Ultra
SCSI 3 (up to 40MB/sec)
Also known as HD (High-Density) 68 & HP (Half-Pitch)
DB68.
Micro 68-Pin Male
16-bit Wide Ultra 2 (LVD)
SCSI 3 (80 to 160MB/sec)
