31-05-2012, 04:52 PM
Small Computer System Interface
[attachment=23751]
introduction
SCSI is actually an acronym for Small Computer System Interface and it is
pronounced as “skuzzy”. It is the second-most popular hard disk interface used in
PCs today. It's a high-speed, intelligent peripheral I/O bus with a device
independent protocol for transferring data between different types of peripheral
devices. The SCSI bus connects all parts of a computer system so that they can
communicate with each other. The bus frees the host processor from the
responsibility of I/O internal tasks. A SCSI bus can be either internal, external, or
cross the boundary from internal to external. The SCSI protocol is a peer-to-peer
relationship: one device does not have to be subordinated to another device in
order to perform I/0 activities. Only two of these devices can communicate on the
bus at any given time.
SCSI-1
SCSI-1 supports transfer rates of up to 5 Mbps and up to 7 devices on an 8-bit
bus (not including the host adapter). The most common types of connectors for
SCSI-1 are the Amphenol 50-pin for external connectivity and 50-pin Dual-Row
Socket F (IDC) connectors for internal connections.
SCSI-2
Approved by ANSI in 1994, SCSI-2 raised the bar to 10 Mbps on a 16 bit bus.
Using the 32 bit bus from the standard, known as "Wide SCSI", the rate increases
to 20 Mbps. SCSI-2 can run as high as 40 Mbps when combining both the Fast
and Wide specifications of the SCSI-2 standard. SCSI-2 usually uses a Micro-D
subminiature 50-pin connector for external cables. Internally, like SCSI-1, it
connects with the same 50-pin IDC connector.
SCSI-3
Primarily implemented in high-end systems, SCSI-3 commonly uses a 68-pin
ribbon cable for in-cabinet connections, and a 68-pin shielded twisted-pair for
external connections. Unlike SCSI-1 and SCSI-2, the internal and external 68-pin
connectors can be interconnected. Although the most common bus width for
SCSI-3 is 16-bit with transfer rates of 20 Mbps, SCSI-3 has specifications for 80
Mbps (Ultra2) and 160 Mbps (Ultra160) throughputs. The Ultra2 and Ultra160
use a technology called Low Voltage Differential (LVD) to achieve higher
speeds at nominal cable distances.
LIMITATIONS OF SCSI 1
It was quite limited, especially by today's standards, and defined only the most
fundamental of SCSI features and transfer modes. It did not include definitions
of a device independent interface. So it wasn’t guaranteed that any device can
be connected.SCSI-1 is now obsolete, and the standard has in fact been
withdrawn by ANSI.
Devices that adhere to the SCSI-1 standard can in most cases be used with host
adapters and other devices that use the higher transfer rates of the more
advanced SCSI-2 protocols, but they will still function at their original slow
speed. Since all SCSI-1 devices are single-ended, they may cause performance
degradation if placed onto a multimode LVD SCSI bus. If you want to run
LVD devices to their full potential, you will want to avoid mixing them with
single-ended devices
SPI-2
SPI-2 doubles bus speed again to the Ultra 2 (Fast-40) SCSI data throughput of
40 Megatransfers/s (80 Mbytes/s for 16-bit).
To attain this speed, a new electrical interface is defined. This interface uses 3 V
logic instead of TTL voltage levels and is known as Low Voltage Differential
(LVD) SCSI. The older TTL based differential SCSI is now called High Voltage
Differential (HVD) and it is not compatible with LVD signals.
SPI-3
SPI-3 again doubles the SCSI bus speed to Ultra 3 (also known as Ultra160 and
Fast-80) providing SCSI bus speeds up to 80 Megatransfers/sec (160 Mbytes/sec
for 16-bit). Double Transition (DT) clocking is used to double the data transfer
rate from Ultra 2 (Fast-40) to Ultra 3 (Ultra 160 or Fast-80) SCSI without having
to increase the clock speed. For this speed, clocking on both the rising and falling
edges of the REQ and ACK clock is required. This is called Double Transition
(DT) clocking and is defined for the 16-bit bus only.
[attachment=23751]
introduction
SCSI is actually an acronym for Small Computer System Interface and it is
pronounced as “skuzzy”. It is the second-most popular hard disk interface used in
PCs today. It's a high-speed, intelligent peripheral I/O bus with a device
independent protocol for transferring data between different types of peripheral
devices. The SCSI bus connects all parts of a computer system so that they can
communicate with each other. The bus frees the host processor from the
responsibility of I/O internal tasks. A SCSI bus can be either internal, external, or
cross the boundary from internal to external. The SCSI protocol is a peer-to-peer
relationship: one device does not have to be subordinated to another device in
order to perform I/0 activities. Only two of these devices can communicate on the
bus at any given time.
SCSI-1
SCSI-1 supports transfer rates of up to 5 Mbps and up to 7 devices on an 8-bit
bus (not including the host adapter). The most common types of connectors for
SCSI-1 are the Amphenol 50-pin for external connectivity and 50-pin Dual-Row
Socket F (IDC) connectors for internal connections.
SCSI-2
Approved by ANSI in 1994, SCSI-2 raised the bar to 10 Mbps on a 16 bit bus.
Using the 32 bit bus from the standard, known as "Wide SCSI", the rate increases
to 20 Mbps. SCSI-2 can run as high as 40 Mbps when combining both the Fast
and Wide specifications of the SCSI-2 standard. SCSI-2 usually uses a Micro-D
subminiature 50-pin connector for external cables. Internally, like SCSI-1, it
connects with the same 50-pin IDC connector.
SCSI-3
Primarily implemented in high-end systems, SCSI-3 commonly uses a 68-pin
ribbon cable for in-cabinet connections, and a 68-pin shielded twisted-pair for
external connections. Unlike SCSI-1 and SCSI-2, the internal and external 68-pin
connectors can be interconnected. Although the most common bus width for
SCSI-3 is 16-bit with transfer rates of 20 Mbps, SCSI-3 has specifications for 80
Mbps (Ultra2) and 160 Mbps (Ultra160) throughputs. The Ultra2 and Ultra160
use a technology called Low Voltage Differential (LVD) to achieve higher
speeds at nominal cable distances.
LIMITATIONS OF SCSI 1
It was quite limited, especially by today's standards, and defined only the most
fundamental of SCSI features and transfer modes. It did not include definitions
of a device independent interface. So it wasn’t guaranteed that any device can
be connected.SCSI-1 is now obsolete, and the standard has in fact been
withdrawn by ANSI.
Devices that adhere to the SCSI-1 standard can in most cases be used with host
adapters and other devices that use the higher transfer rates of the more
advanced SCSI-2 protocols, but they will still function at their original slow
speed. Since all SCSI-1 devices are single-ended, they may cause performance
degradation if placed onto a multimode LVD SCSI bus. If you want to run
LVD devices to their full potential, you will want to avoid mixing them with
single-ended devices
SPI-2
SPI-2 doubles bus speed again to the Ultra 2 (Fast-40) SCSI data throughput of
40 Megatransfers/s (80 Mbytes/s for 16-bit).
To attain this speed, a new electrical interface is defined. This interface uses 3 V
logic instead of TTL voltage levels and is known as Low Voltage Differential
(LVD) SCSI. The older TTL based differential SCSI is now called High Voltage
Differential (HVD) and it is not compatible with LVD signals.
SPI-3
SPI-3 again doubles the SCSI bus speed to Ultra 3 (also known as Ultra160 and
Fast-80) providing SCSI bus speeds up to 80 Megatransfers/sec (160 Mbytes/sec
for 16-bit). Double Transition (DT) clocking is used to double the data transfer
rate from Ultra 2 (Fast-40) to Ultra 3 (Ultra 160 or Fast-80) SCSI without having
to increase the clock speed. For this speed, clocking on both the rising and falling
edges of the REQ and ACK clock is required. This is called Double Transition
(DT) clocking and is defined for the 16-bit bus only.