| N |
| Narrow
SCSI (vs. Wide) |
This
is the term attributed to today's 8 bit standard SCSI devices.
This term is necessary to distinguish today's 8-bit SCSI devices
to 16-bit Wide SCSI devices. |
| |
| O |
| |
|
| P |
| Parity |
A
form of data redundancy used by RAID levels 2, 3, 4 and 5 to
recreate the data of a failed drive in a disk array. |
| PROM |
An
acronym for Programmable Read Only Memory. This is a version
of a ROM that is programmable. |
| Plug
and Play |
A
standard, pioneered by Microsoft and endorsed by industry leaders.
This standard hopes to address the problems of adding I/O adapters
to a PC computer system. Adapters designed to the Plug and Play
standard will self configure, and automatically resolve system
resources such as interrupts (IRQ), DMA, port addresses, and
BIOS addresses. |
| |
| Q |
| |
|
| R |
| RAID |
Redundant array
of inexpensive disks. The term coined in 1987 by researchers
at the University of California at Berkeley to describe a
series of redundant architectures used in fault-tolerant disk
arrays (RAID levels 1 through 5).
| Level
|
Description
|
Speed |
Redundancy |
| 0 |
Striping |
Breaks
the data stream into small chunks and writes each piece
to a different drive. |
Very
fast |
About
the same as no RAID |
| 1 |
Mirroring |
Data
sent to one drive is duplicated on extra drive. |
Fast
reads, slower writes |
Very
high |
| 3 |
Parity |
The
data stream is split between several disks (as in RAID
0) with an extra disk providing error protection. |
Very
fast reads, fast writes |
Medium |
| 5 |
Parity |
The
data stream is split between several disks with error
protection alternating writes between disks. |
Very
fast reads, fast writes |
Medium |
|
| |
| S |
| SCAM |
This
is also known as Plug and Play for SCSI. Using this specification,
the SCSI host adapter is able to automatically select the SCSI
ID of itself and attached SCSI devices. It can also enable/disable
termination as required to properly terminate the SCSI bus.
This is an attempt to make SCSI easier to use, since the user
no longer has to worry about setting SCSI ID jumpers or SCSI
bus termination. |
| SCSI |
A
PC bus interface standard that defines standard physical and
electrical connections for devices. SCSI provides a standard
interface that enables many different kinds of devices, such
as disk drives, magneto optical disks, CD-ROM drives, and tape
drives to interface with the host computer. |
| SCSI-1 |
8 bit SCSI, Single-ended
open collector drivers with Asynchronous transfer for all
commands and data transfers. Bipolar technology for the drivers
and receivers, bus DC loads a major consideration, passive
termination.
Asynchronous
or synchronous transfers up to 5 megabytes per second, below
defines how it is used in the different generations of the
SCSI standards
|
| SCSI-2 |
8 bit SCSI asynchronous
commands with Synchronous data transfer rates up to 10 megabytes
per second. Bipolar technology with Open collector or active
negation single ended bus transceivers, passive or active
termination, and Differential (Now called HVD – High Voltage
differential) bus transceivers based on EIA 485 allowed in
SCSI-2.
|
| Striping |
Spreading
data evenly over multiple disk drives to enhance performance.
Data striping can be performed on a bit, byte or block basis
for optimum application performance. |
| Synchronous
Transfer |
A
method of SCSI data transfer. With this type of data transfer,
the SCSI host adapter and the SCSI device agree to a transfer
rate that both support (this is known as synchronous negotiation).
With this type of data transfer method, transfer rates of 5
MBytes/sec or 10 MBytes/sec (for FAST SCSI) are common. |
| |
| T |
| Tagged
Queueing |
A
SCSI-2 feature that increases performance on SCSI disk drives.
With tagged queuing, the host adapter, the host adapter driver,
and the hard disk drive work together to increase performance
by reordering the requests from the host adapter to minimize
head switching and seeking. For example, the host adapter may
ask for the following data in the following order; LBA 0, 1,
101, 102, 5, 6 (LBA = logical block address, or a byte of data)
|
| Termination |
A
physical requirement of the SCSI bus. The first and last devices
on the SCSI bus must have terminating resistors installed, and
the devices in the middle of the bus must have terminating resistors
removed. |
| |
| U |
| Ultra
SCSI |
Data
transfers 20 megabytes per second (8-bit bus). 40MB/sec with
a 16-bit bus. |
| Ultra2
SCSI |
Data
transfers 40 megabytes per second (8-bit bus). 80MB/sec with
a 16-bit bus. |
| Ultra3
SCSI |
Data
transfers 160 megabytes per second. 16 bit SCSI asynchronous
commands with Synchronous data transfer rates up to 160 megabytes
per second. LVD (Low Voltage Differential allowed in SCSI SPI-3.
There are a lot of changes with SPI-3; High voltage differential
(HVD SCSI) and the 32 bit data bus with the Q cable are obsolete,
clocking is defined for LVD on both the rising and falling edges
of the REQ/ACK clock – Double Transition (DT) defined, Cyclic
Redundancy Check (CRC) defined, Domain Validation defined in
SPI-3 and SPC-x, Packetized defined and Quick Arbitration Defined. |
| Ultra160
SCSI |
Ultra160
SCSI is a specific implementation of Ultra3 that includes the
three key components of Ultra3 SCSI: Fast-80DT, CRC and Domain
Validation. The maximum Ultra160 SCSI data transfer rates are
160MBytes/second. |
| Ultra160+ |
All
the features in Ultra160 SCSI adding support for packetization
and quick arbitration. These are all the features defined in
the Ultra3 specification. |
| |
| V |
| |
|
| |
| W |
| Wide
SCSI |
Wide
SCSI had a bus width of 16 bits, whereas narrow SCSI has a bus
width of 8 bits. The data transfer rate that can be realized
depends not only on the bus width but also on the SCSI standard
that is being used on the implementation (i.e., Fast SCSI can
transfer at 10Mbytes/sec and Fast Wide SCSI can transfer at
20Mbytes/sec). |
| Wide
Ultra2 SCSI |
Data
transfers 80 megabytes per second. |
| |
| X |
| |
|
| Y |
| |
|
| Z |
| |
|
