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Fuzzy about SCSI?
SCSI
(Small Computer Systems Interface) is a high-speed, general-purpose peripheral
interface. Over the years, it has expanded to offer a variety of data transfer
rates and an increased bus bandwidth. Today, SCSI is now more a family of
interfaces than a single interface.
SCSI
became well known in the late 1980’s, mostly for its use in high-end workstations
and server environments. SCSI quickly replaced ESDI and SMD interfaces as
prices and performance in SCSI technology improved.
To
really understand SCSI you have to realize that SCSI has a variety of attributes
and that each can integrate with any other attribute. These attributes include:
1) Bandwidth, 2) Interface, 3) Speed, 4) Interconnect (connector type).
Bandwidth:
The bandwidth is how many bits can be transmitted on the bus at the same
time. SCSI has only two bandwidth options, Narrow 8-Bit and Wide 16-Bit.
The number of bits on the bus also define how many devices can technically
be added to the SCSI bus. On 8-Bit busses, 8 devices can be added (including
the host adapter) and on 16-Bit busses up to 16 devices can be used.
Interface: Single-ended, Differential (High Voltage Differential), LVD (Low Voltage Differential).
- Single-ended:
The most common SCSI interface in use today. Single-ended SCSI uses half
the cable wires for data/control and the other half as a common ground point.
Pros: low cost, widely deployed. Cons: limited SCSI cable length and limited
number of devices that can be reliably added to the bus due to cable length
constraints and bus noise.
- Differential:
Also known as High Voltage Differential, solves the problems of single-ended
SCSI by separating each of the common ground lines with its own distinct
matching data/control line. Pros: longer SCSI cable length, increased number
of devices can be reliably added, data transfer rates run faster due to reduced
noise. Cons: not as widely deployed, more costly, fading technology with
the addition of LVD.
- Low
Voltage Differential: Commonly known as LVD, the newest addition to the SCSI
interface has all the advantages of both Single-ended and High Voltage Differential.
LVD is being rapidly made the de facto standard in high-performance SCSI.
Pros: increasingly lower cost, becoming widely deployed, long cable distances
compared to single-ended, supports LVD and single-ended devices on the same
bus, is compatible with older SCSI technologies. Cons: not yet widely available
on all computing platforms.
Important Note: Ultra2 speed IS automatically an LVD interface. An LVD interface IS NOT automatically Ultra2 speed.
Speed: This is how fast the data transfers on the SCSI bus and is closely tied to Bandwidth.
- Narrow 8-Bit SCSI-1 up to 5MB/Sec
- Narrow 8-Bit SCSI-2 up to 10MB/Sec
- Wide 16-Bit SCSI-2 up to 20MB/Sec
- Narrow 8-Bit Ultra SCSI up to 20MB/Sec
- Wide 16-Bit Ultra SCSI up to 40MB/Sec
- Narrow 8-Bit Ultra2 SCSI up to 40MB/Sec
- Wide 16-Bit Ultra2 SCSI up to 80MB/Sec
- Narrow 8-Bit Ultra3 SCSI up to 80MB/Sec*
- Wide 16-Bit Ultra3 SCSI up to 160MB/Sec*
*Based on preliminary specifications, Ultra3 SCSI at the time of this document is still in development.
Interconnect:
- DB25, looks like my parallel port
- Centronics, Blue-Ribbon, looks like my printer connector
- IDC, dual-row header, two-row rectangle with a notch
- DB50, three rows of pins in a D style
- HD50, High-Density 50, mini 50, two rows of pins in a D style
- HD68, High-Density 68, mini 68, looks like and HD50 but longer
- SCA, single-connect, 80-pin single-connect – integrates power, SCSI ID, and data/control into one connector
- VHDCI, ultra mini 68, Wow it’s really small let me get my glasses
Each
of the attributes in Bandwidth, Speed, Interface, and Interconnect must be
used to accurately describe a SCSI device. For instance you will commonly
find “Wide, Ultra SCSI, Single-Ended Interface, HD68” on a wide variety of
disk drives and other peripherals. You may however never see “Narrow, SCSI-2,
LVD, Centronics” even though it is technically possible. To insure you receive
the right device to connect to your computer, use all the attributes when
describing your interface.
In
general SCSI is a very economical and high-performance method to connect
a variety of devices to your computer system. SCSI has performance advantages
over IDE/ATA/UltraATA by not using system CPU resources, SCSI has cost advantages
over upcoming Fiber Channel technologies, and SCSI has the market advantage
by being supported and deployed by a huge number of manufacturers worldwide.
In short, SCSI is a great interface for nearly every application.
But, there is always something newer around the corner……..
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