SCSI (Pronounced as Skuzzy) is an Acronym for Small Computer System Interface. It is a set of standards for connecting and transferring data between Computer and Peripheral devices. It was developed by Shugart Associates Company in 1981 as a universal and intelligent disk drive interface.
This Interface is generally used for connecting Hard Disk Drives and Tape Drives but can also be used to connect wide range of other devices. This Interface is derived from Shugart Associates System Interface (SASI pronounced as sassy). SASI Controller works as bridge between Disk Drive’s Low level interface and Host Computer. Larry Boucher is considered to be the father of SASI and SCSI. Up to February 1982, ANSI developed this specification as SASI and “Shugart Associates System Interface”. Committee documenting this Standard did not allow it to be named after a company name and after a full day discussion named as SCSI (Small Computer System Interface).
Some Main Features of this Interface are as Follows:
1) SCSI is an intelligent, peripheral, buffered, peer to peer interface
2) Concept of initiator and Target: There are two types of devices on SCSI bus i) I/O Initiators and ii) I/O Targets. I/O Initiators are the devices that start the I/O process and Target are those devices that accomplish this task.
3) Device Addressing: SCSI devices connected to the bus must use unique identification addresses, each one with one ID bit assigned to it.
2) Concept of initiator and Target: There are two types of devices on SCSI bus i) I/O Initiators and ii) I/O Targets. I/O Initiators are the devices that start the I/O process and Target are those devices that accomplish this task.
3) Device Addressing: SCSI devices connected to the bus must use unique identification addresses, each one with one ID bit assigned to it.
4) Use of Logical Units (LUN): each device on the SCSI bus has its own unique address – the SCSI ID. In addition to this, each Target is also sub-divided into Logical Units (LUN). The maximum number of Logical Units in SCSI-2 is eight: LUN-0 to LUN-7. The SCSI-3 allows an almost unlimited number of LUNs (SAM reserves a 64-bit field for LUN). The actual number of LUNs used depends on each specific carrier protocol.
5) It Hides Complexity of Physical format and every device connected to this works in same manner.
6) 8 Devices can be connected to an 8 Bit SCSI Bus and Max. Of 16 Devices can be connected to a 16 Bit SCSI Bus.
7) There can be any no. of Hosts and I/O devices but there should be at least one host.
8) SCSI makes use of Handshake Signals with devices for Data Transmission .Two types of handshaking protocols are used for controlling the transfer of information i.e. Synchronous and Asynchronous. The two modes differ in the way the strobe signals REQ and ACK handshake.
9) Parity Checking Feature is available in SCSI-1 and SCSI-2.
10) SCSI is both a bus hardware specification and a command set to optimize use of that bus.
11) Ability to Disconnect and Reconnect: This feature allows slow operations (like seek in disks, or rewind in tapes) to be executed off line. Using disconnect and reconnect, several operations in a system may occur concurrently.
12) Logical Block Addressing (LBA): Uniformity of data addressing in HDD is maintained by concept of logical blocks. If the system wants to read data from a particular Logical Block Address (LBA), it does not need to know about the physical geometry or layout of the device. All knowledge of the physical details (e.g. track, cylinder, head, and sector address) is embodied within the device itself, where the logical address is internally converted to a physical address.
10) SCSI is both a bus hardware specification and a command set to optimize use of that bus.
11) Ability to Disconnect and Reconnect: This feature allows slow operations (like seek in disks, or rewind in tapes) to be executed off line. Using disconnect and reconnect, several operations in a system may occur concurrently.
12) Logical Block Addressing (LBA): Uniformity of data addressing in HDD is maintained by concept of logical blocks. If the system wants to read data from a particular Logical Block Address (LBA), it does not need to know about the physical geometry or layout of the device. All knowledge of the physical details (e.g. track, cylinder, head, and sector address) is embodied within the device itself, where the logical address is internally converted to a physical address.
SCSI Origin
Before 1980 only mainframe and mini computers were used. The capabilities of microcomputers of that day were limited; they used an 8-bit microprocessor, 64K of main memory, and two floppy disks for external storage. But with the invention of new application softwares there was a need to improve performance and storage capacity of a Computer System. The first Winchester disk drive was introduced with a maximum transfer rate of 600 KBps and up to 10-MByte capacity. The Seagate ST506 scheme, used as a dedicated interface, was not satisfactory for long. With the need for higher performance, a universal interface SCSI designed.
SCSI-1:
It is the original SCSI specification developed in 1986. There was a serious interoperability problem between products from different vendor’s .When SCSI finally approved in 1986; the Common Command Set (CCS) was proposed to resolve compatibility problems. It was a subset of standard and did not allow exceptions. The first version of SCSI was already out and hence named it as SCSI-1.
The main Characteristics of SCSI-1 are as follows:
1) Cable Length was very short (1 meter or less) and with only two SCSI Connectors/Devices.
2) Only one initiator and one target were supported.
3) Data transfer mode was asynchronous only.
4) Data Transfer rate was only 1MBps or less.
5) No disconnect, No Parity and no arbitration signals were used.
6) Single Ended Interfaces were used.
7) 6 Byte CDB (Command Descriptor Block: The Target takes over and requests a command from the Initiator. The Initiator responds by sending the command code (Command Descriptor Block - CDB) to be executed.)
8) Overhead of 80% or even more.
It is the original SCSI specification developed in 1986. There was a serious interoperability problem between products from different vendor’s .When SCSI finally approved in 1986; the Common Command Set (CCS) was proposed to resolve compatibility problems. It was a subset of standard and did not allow exceptions. The first version of SCSI was already out and hence named it as SCSI-1.
The main Characteristics of SCSI-1 are as follows:
1) Cable Length was very short (1 meter or less) and with only two SCSI Connectors/Devices.
2) Only one initiator and one target were supported.
3) Data transfer mode was asynchronous only.
4) Data Transfer rate was only 1MBps or less.
5) No disconnect, No Parity and no arbitration signals were used.
6) Single Ended Interfaces were used.
7) 6 Byte CDB (Command Descriptor Block: The Target takes over and requests a command from the Initiator. The Initiator responds by sending the command code (Command Descriptor Block - CDB) to be executed.)
8) Overhead of 80% or even more.
SCSI-2
When SCSI-1 and CCS were combined we got the Standard SCSI-2. It turned out that the SCSI-2 specifications document grew to more than double the size of SCSI-1. The new draft was submitted to ANSI for approval in 1990, and was finally released in January 1994, as revision "10L" with almost 500 pages.
Following are the main improvements in SCSI-2:
1) Higher Performance than SCSI-1.
2) Increased Transfer rate of 10 MHz’s.
3) Overhead reduced to 30% or less.
4) Synchronous Data Transfer mode support.
5) Single Ended (SE) and Differential (HVD) Interface support.
6) 4 Byte wide SCSI Bus (While SCSI-1 was only 1 byte wide) . This bus not gained much popularity and now only 2 byte wide buses are used with 20MBps burst data transfer rate with diversity of peripheral devices.
7) Improved compatibility feature.
8) Possibility of building more complex configurations using a mix of peripherals with differing functions and performances, all on the same SCSI cable.
9) Execution of higher intelligence functions, and executing them concurrently (Disconnect, Command Queuing).
10) Arbitration, Parity, Error reporting & classification, new commands and extended message system added.
SCSI-3:
When SCSI-1 and CCS were combined we got the Standard SCSI-2. It turned out that the SCSI-2 specifications document grew to more than double the size of SCSI-1. The new draft was submitted to ANSI for approval in 1990, and was finally released in January 1994, as revision "10L" with almost 500 pages.
Following are the main improvements in SCSI-2:
1) Higher Performance than SCSI-1.
2) Increased Transfer rate of 10 MHz’s.
3) Overhead reduced to 30% or less.
4) Synchronous Data Transfer mode support.
5) Single Ended (SE) and Differential (HVD) Interface support.
6) 4 Byte wide SCSI Bus (While SCSI-1 was only 1 byte wide) . This bus not gained much popularity and now only 2 byte wide buses are used with 20MBps burst data transfer rate with diversity of peripheral devices.
7) Improved compatibility feature.
8) Possibility of building more complex configurations using a mix of peripherals with differing functions and performances, all on the same SCSI cable.
9) Execution of higher intelligence functions, and executing them concurrently (Disconnect, Command Queuing).
10) Arbitration, Parity, Error reporting & classification, new commands and extended message system added.
SCSI-3:
To satisfy today’s requirement faster and enhanced standards are developed, this standard is called SCSI-3. This interface is of two types:
i) SCSI-3 Parallel Interface.
ii) SCSI-3 Serial Interface.
Below given Table illustrates All SCSI standards and their specifications
i) SCSI-3 Parallel Interface.
ii) SCSI-3 Serial Interface.
Below given Table illustrates All SCSI standards and their specifications
Interface
|
Alternative
names |
Specification
document |
Connector Used
|
Width
(bits) |
Bus Clock
|
Throughput (MB/s)
|
Throughput (Mbit/s)
|
Length
(single ended) |
Length LVD
|
Length HVD
|
Devices Supported
|
SCSI-1
|
Narrow SCSI
|
SCSI-1 (1986)
|
IDC50; Centronics C50
|
8
|
5 MHz
|
5 MB/s
|
40 Mbit/s
|
6 m
|
NA
|
25 m
|
8
|
Fast SCSI
|
SCSI-2 (1994)
|
IDC50; Centronics C50
|
8
|
10 MHz
|
10 MB/s
|
80 Mbit/s
|
3 m
|
NA
|
25 m
|
8
| |
Fast-Wide SCSI
|
SCSI-2;
SPI-5 (INCITS 367-2003) |
2 x 50-pin (SCSI-2);
1 x 68-pin (SCSI-3) |
16
|
10 MHz
|
20 MB/s
|
160 Mbit/s
|
3 m
|
NA
|
25 m
|
16
| |
Ultra SCSI
|
Fast-20
|
SPI-5 (INCITS 367-2003)
|
IDC50
|
8
|
20 MHz
|
20 MB/s
|
160 Mbit/s
|
1.5 m
|
NA
|
25 m
|
8
|
3 m
|
NA
|
NA
|
4
| ||||||||
Ultra Wide SCSI
|
SPI-5 (INCITS 367-2003)
|
68-pin
|
16
|
20 MHz
|
40 MB/s
|
320 Mbit/s
|
NA
|
NA
|
25 m
|
16
| |
1.5 m
|
NA
|
NA
|
8
| ||||||||
3 m
|
NA
|
NA
|
4
| ||||||||
Ultra2 SCSI
|
Fast-40
|
SPI-5 (INCITS 367-2003)
|
50-pin
|
8
|
40 MHz
|
40 MB/s
|
320 Mbit/s
|
NA
|
12 m
|
25 m
|
8
|
Ultra2 Wide SCSI
|
SPI-5 (INCITS 367-2003)
|
68-pin; 80-pin (SCA/SCA-2)
|
16
|
40 MHz
|
80 MB/s
|
640 Mbit/s
|
NA
|
12 m
|
25 m
|
16
| |
Ultra3 SCSI
|
Ultra-160; Fast-80 wide
|
SPI-5 (INCITS 367-2003)
|
68-pin; 80-pin (SCA/SCA-2)
|
16
|
40 MHz DDR
|
160 MB/s
|
1280 Mbit/s
|
NA
|
12 m
|
NA
|
16
|
Ultra-320 SCSI
|
Ultra-4; Fast-160
|
SPI-5 (INCITS 367-2003)
|
68-pin; 80-pin (SCA/SCA-2)
|
16
|
80 MHz DDR
|
320 MB/s
|
2560 Mbit/s
|
NA
|
12 m
|
NA
|
16
|
Ultra-640 SCSI
|
Ultra-5; Fast-320
|
SPI-5 (INCITS 367-2003)
|
68-pin; 80-pin
|
16
|
160 MHz DDR
|
640 MB/s
|
5120 Mbit/s
|
NA
|
10 m
|
NA
|
16
|
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