The SCSI Interface
SCSI - Small Computer
Systems Interface
SCSI is an industry-standard input/output bus interface for "small"
(non-mainframe) computers. SCSI defines standards for the computer hardware,
software, and communication to connect SCSI disk drives, CD-ROM drives,
scanners, and other kinds of devices to a common SCSI bus to enable them
to send commands and data to each other
Max. Transfer
Rates |
SCSI-1
Standard SCSI |
SCSI-2
Fast SCSI |
SCSI-3
Ultra SCSI |
Ultra2 SCSI
|
Bus
Width |
Max.Nr.
Devices |
Standard SCSI |
5 Mbit/s |
10 Mbit/s |
20 Mbit/s |
40 Mbit/s |
8-bit |
7 |
Wide SCSI |
10 Mbit/s |
20 Mbit/s |
40 Mbit/s |
80 Mbit/s |
16-bit |
15 |
SCSI-1
SCSI-1 is the original SCSI specification and supports:
- Upto seven 8-bit devices
- WORM (Write Once Read Many) devices
SCSI-2
SCSI-2 specification added these features to SCSI-1:
- Small, high-density connectors
- CD-ROM drives and scanners
- SCSI bus parity checking
- Fast SCSI data transfer
- Wide SCSI 16-bit bus
- Tagged queuing
SCSI-3
SCSI-3 specification added these features to SCSI-2:
- Very fast Ultra SCSI data transfer
- Support for fiber channel hardware
- Support for serial channel hardware
- P-cable wide bus, supporting up to 16/32 SCSI devices on a single bus
Fast SCSI
Fast SCSI allows data to be transferred at up to 10 Mbytes per second
on the 8-bit SCSI bus
Most currently manufactured SCSI disk devices support Fast SCSI data
transfer. Fast SCSI devices require synchronous data transfer. The maximum
SCSI bus length for Fast SCSI devices is 3 meters
Wide SCSI
Wide SCSI allows the connection of upto fifteen 8-bit or 16-bit SCSI devices
on one bus. The 16-bit SCSI devices can transmit data twice as fast as
8-bit SCSI devices. Wide SCSI allows data transfer rates upto 20 Mbytes
per second
TwinChannel SCSI
TwinChannel SCSI host adapters have two SCSI channels (buses) on one host
adapter board. Each channel can support upto seven SCSI devices (fifteen
devices for Wide SCSI host adapters). This allows the connection of twice
as many SCSI devices as a single-channel SCSI board while using only one
of the computer's expansion slots
On TwinChannel host adapters, one of the SCSI channels usually has connectors
for internal SCSI devices and external SCSI devices, while the other channel
supports only internal SCSI devices
Ultra SCSI
Ultra SCSI (also called Fast-20 or Double Speed SCSI) defines how to move
data at a rate of 20 MBytes per second on an 8-bit SCSI bus or 40 MBytes
per second on a 16-bit Wide SCSI bus
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