Glossary of SCSI
terms
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- ADAPTEC EZ-SCSI
- A user-friendly software program that automatically installs
SCSI devices such as fixed disks and CD-ROM drives on a PC.
Adaptec EZ-SCSI copies the required software programs to the PC's
fixed disk and edits the configuration files so the host adapter
can access the devices.
- ACK
- An acronym for Adaptec Cable Kit.
- ACTIVE NEGATION:
- Provides better noise immunity on the complex SCSI bus.
- ACTIVE TERMINATION:
- An active terminator actually has one or more voltage
regulators to produce the termination voltage, rather than using
resistor voltage dividers.
This is a passive terminator: TERMPWR --/\/\/\/--+--/\/\/\/-- GND
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SCSI signal
Notice that the termination voltage is varies with the voltage
on the TERMPWR line. One voltage divider (two resistors) is used
for each SCSIsignal.
An active terminator looks more like this (supply filter caps
omitted): +---------+
| TERMPWR --| in out |--+--/\/\/\/--SCSI signal
| gnd | |
+---------+ |
| +--/\/\/\/--SCSI signal
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GND ----+ |
+--/\/\/\/--SCSI signal
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etc.
Assuming that the TERMPWR voltage doesn't drop below the
desired termination voltage (plus the regulator's minimum drop),
the SCSI signals will always be terminated to the correct voltage
level.
- AHA
- An acronym for Adaptec Host Adapter.
- AIC
- An acronym for Adaptec Integrated Circuit
- AIC-6260
- Original version of the AIC-6360.
- AIC-6360
- Second generation Adaptec single chip non-bus mastering ISA to
SCSI host adapter (compatible with the first version, the
AIC-6260). This chip is found on some Adaptec host adapters, such
as the AVA-1505, AVA-1515, AHA-1520A, AHA-1522A, and the AMM-1570.
Adaptec also licenses the chip to motherboard vendors and other
I/O card manufacturers.
- AIC-7770
- An Adaptec single chip RISC based bus mastering EISA to SCSI
host adapter. This chip is found on some Adaptec host adapters,
such as the AHA-2740A and the AHA-2840A series. Adaptec also
licenses the chip to motherboard vendors and other I/O card
manufacturers.
- AIC-7870
- An Adaptec single chip RISC based bus mastering PCI to SCSI
host adapter. This chip is found on some Adaptec host adapters,
such as the AHA-2940 and the AHA-2940W. Adaptec also licenses the
chip to motherboard vendors and other I/O card manufacturers.
- APA
- An acronym for Adaptec Portable Adapter.
- ASPI
- Advanced SCSI programming Interface. A standard SCSI software
interface that acts as a liaison between host adapters and SCSI
device drivers. ASPI enables host adapters and device drivers to
share a single SCSI hardware interface.
- ASPI MANAGER
- A software module that provides an interface between ASPI
modules, a host adapter board, and the SCSI devices connected to
the adapter. A single ASPI manager can handle multiple
Input/Output requests from multiple ASPI modules. ASPI managers
are put requests from multiple ASPI modules. ASPI managers are
written for a specific operating system - such as DOS, OS/2 or
Unix-and a specific family of host adapter boards. The Adaptec
EZ-SCSI ASPI managers for DOS are ASPI4DOS, ASPIEDOS and ASPI2DOS.
- ASPI MODULE
- Device-level code specific to a particular kind of SCSI device
that communicates with the ASPI manager. The Adaptec EZ-SCSI ASPI
modules are ASPIDISK. ASPICD.
- ASW
- An acronym for Adaptec Software.
- ASYNCHRONOUS DATA TRANSFER
- A method of SCSI data transfer. This is the type of transfer
rate originally introduced with SCSI 1. With this type of transfer
method, transfer rates of 2 MBytes/sec are common. See also
SYNCHRONOUS TRANSFER.
Is SYNCHRONOUS faster than
ASYNCHRONOUS?
Asynchronous is faster on short cables,
while synchronous is faster on long cables. The reason has to do
with the propagation delay of the cable; the turn around time of
the silicon; and the interlocked nature of the asynchronous
handshake.
1) We have measured propagation delays from various cables and
found an average of 1.7 nanoseconds per foot, which is roughly
5.25 ns per meter.
2) The turn-around time is the amount of time the SCSI chip
takes to change an output in response to an input. If REQ is an
input then ACK is an output. Or if ACK is an input then REQ is an
output. Typical turn-around time for the 53C90 is 40 nanoseconds.
3) The asynchronous transfer uses an interlocked handshake
where a device cannot do the next thing until it receives positive
acknowledgment that the other device received the last thing.
- AVA
- An acronym for Adaptec Value Adapter.
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- BIOS
- An acronym for Basic Input/Output System. This is usually an
EPROM with computer program instructions in it. A motherboard BIOS
(usually by companies such as Phoenix, Award, and AMI) controls
the basic functions of the computer (such as controlling the
keyboard, monitor, etc.).
With a SCSI host adapter, the BIOS is used to control SCSI hard
disk drives and perform the boot function. If a host adapter does
not have a BIOS, then hard disk drives controlled by that host
adapter cannot be used to boot from (booting must be done from
another source, such as floppy, IDE, or another SCSI host adapter
with a BIOS). The BIOS must be enabled in order to function (e.g.
a host adapter with a BIOS that is disabled acts the same as a
host adapter without a BIOS).
The BIOS can also contain useful software utilities, such as
Adaptec's SCSISelect utility, which can be used to change the host
adapter settings, format disks, and run simple SCSI diagnostics.
For more information on SCSI host adapter BIOS, see the
'Troubleshooting Guide' in section 6 of this guide.
- BLOCK COMMAND
- Automatically buffers the messages, command bytes and status
bytes.
- BOOTING
- Booting is a process by which a computer starts and
automatically loads the operating system.
- BUS
- A pathway for data in a computer system. All PCs have an
expansion bus, which is designed to host add-on (expansion)
devices, such as modems, adapter boards and video adapters.
Expansion devices use the bus to send data to and receive data
from the PC's CPU or memory. ISA, EISA and Micro Channel are the
major bus standards used in PC's.
- BUS MASTERING
- A high performance method of data transfer in which the host
adapter's on-board processor handles the transfer of data directly
to and from a computer's memory without intervention from the
computer's microprocessor. This is the fastest method of data
transfer available for multitasking operating systems. Adaptec's
AHA-1540, AHA-1640, and AHA-1740 series host adapters use bus
mastering. (Also called Bus Master DMA or First Party DMA.)
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- COMMAND QUEUEING:
- In SCSI-1, initiators were limited to one command per LUN e.g.
a disk drive. Now up to 256 commands can be outstanding to one
LUN. The target is allowed to re-sequence the order of command
execution to optimize seek motions. Queued commands require Tag
messages which follow the Identify.
- COMMAND SWAPPING
- A SCSI host adapter feature that allows up the host adapter to
support up to 255 simultaneous commands. Without this feature,
AIC-7770 based host adapters support up to four simultaneous
commands.
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- DEVICE DRIVER
- A software program that enables a PC to communicate with
peripheral devices such as fixed disk drives and CD-ROM drives.
Each kind of device requires a different driver. Device driver
programs are stored on a PC's fixed disk and are loaded into
memory at boot time.
- DIFFERENTIAL
- A term referring to the electrical characteristics of the
signals used on the SCSI bus interface. Differential signals
occupy two conductors with a positive (+) and negative (-)
polarity component of the signal. This minimizes the effect of
common mode signal noise and allow the SCSI bus to operate
reliably over greater distances at a higher speed.
- DISK ARRAY
- Two or more disks grouped together to appear as a single disk
to the host system.
- (DMA) DIRECT MEMORY ACCESS
- A mechanism that allows hardware control of the transfer of
streams of data to or from the main memory of a computing system.
The mechanism may require setup by the host software. After
initialization, it automatically sequences the required data
transfer and provides the necessary address information.
- DOS PARTITION
- A section of a disk storage device, created by the DOS FDISK
program, in which data and/or software programs are stored.
Computers have a primary DOS partition that contains the special
files needed to boot the computer. A computer's disk devices may
also have extended DOS partitions. Each DOS partition is assigned
a unique drive letter, such as C or D. A single disk device can
have multiple partitions. After you have installed EZ- SCSI you
can use the AFDISK utility program to partition disk devices.
- DOUBLE WORD PIO TRANSFERS
- This is a type of data transfer done by 386 and newer
microprocessors. With the Adaptec AIC-6360 based host adapters
(AHA-1510A/1520A/1522A), this can result in up to a 20% increase
in performance over the AIC-6260 based host adapters.
- DOWN TIME
- A time interval when a network cannot be used due to equipment
failure or other cause.
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- EEPROM
- An acronym for Electrically Erasable Programmable Read Only
Memory. These devices can be erased instantly.
- EISA (Extended Industry Standard Architecture)
- A computer bus standard compatible with ISA; a 32-bit data
path (bus).
- ENHANCED MODE
- The operation mode of the AHA - 1740A/1742A/1744 that takes
full advantage of the EISA addressing range and register set.
- EMBEDDED
- With reference to operating system application and utility
software, does not require additional diskettes.
- EPROM
- An acronym for Erasable Programmable Read Only Memory. These
devices can be erased by placing them under an ultraviolet light
for several minutes. They can then be reused.
- EZ-SCSI
- Adaptec software package for quick and easy installation. Also
contains useful software utilities.
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- FAST SCSI
- Provides forperformance and compatibility enhancements to
SCSI-1 by increasing the maximum synchronous data transfer rate on
the SCSI bus from 5 Mbytes/sec to 10 Mbytes/sec.
There are 2 handshaking modes on the SCSI bus, used for
transferring data: ASYNCHRONOUS and SYNCHRONOUS. ASYNCHRONOUS is a
classic Req/Ack handshake. SYNCHRONOUS is "sort of" Req/Ack, only
it allows you to issue multiple Req's before receiving Ack's. What
this means in practice is that SYNCHRONOUS transfers are approx 3
times faster than ASYNCHRONOUS.
SCSI-1 allowed asynchronous transfers at up to 1.5 Mbytes/Sec
and synchronous transfers at up to 5.0 Mbytes/Sec.
SCSI-2 had some of the timing margins "shaved" in order that
faster handshaking could occur. The result is that asynchronous
transfers can run at up to 3.0 bytes/Sec and synchronous transfers
at up to 10.0 Mbytes/Sec. The term "FAST" is generally applied to
a SCSI device which can do syncrhonous transfers at speeds in
excess of 5.0 Mbytes/Sec. This term can only be applied to SCSI-2
devices since SCSI-1 didn't have the timing margins that allow for
FAST transfers.
A 10 MHz transfer rate for SCSI came out of a joint effort with
the IPI (Intelligent Peripheral Interface) committee in ASC
X3T9.3. Fast SCSI achieves 10 Megabytes/second on the A-Cable and
with wider data paths of 16- and 32-bits can rise to 20
Megabytes/second and even 40 Megabytes/second. However, by the
time the market starts demanding 40 Megabytes/second it is likely
that the effort to serialize the physical interface for SCSI-3
will attract high-performance SCSI users to the Fibre Channel.
A word of caution. At this time the fast parameters cannot be
met by the Single Ended electrical class, and is only suitable for
Differential. One of the goals in SCSI-3 is to identify the
improvements needed to achieve 10 MHz operation with Single Ended
components.
- FULL SCSI
- A SCSI solution that includes BIOS and support software to
provide boot capability for hard disk drives, support for drives
larger than 1 GB, and full compatibility with removable media
products (hard drives, optical drives, tape drives, and Floptical
drives).
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- h
- This stands for hex, or hexadecimal. This is a counting system
commonly used in computers. It is based on 16 instead of 10
(decimal). An 'h' after a number indicates that this is a hex
number.
- HOST
- A microcomputer in which a host adapter is installed. The host
uses software to request the services of the host adapter in
transferring information to and from peripheral devices attached
to the SCSI bus connector of the host adapter.
- HOST ADAPTER
- A printed circuit board that installs in a standard
microcomputer and provides a SCSI bus connection so that SCSI
devices can be connected to the microcomputer.
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- IBM PC AT COMPATIBLE
- Any computer system that emulates exactly the IBM PC AT and
that uses an ISA backplane bus.
- INDUSTRY STANDARD ARCHITECTURE
- The IBM PC AT functions have been duplicated by a number of
manufacturers. All the IBM PC AT compatible machines use a
backplane bus that very closely emulates the function of the
backplane bus of the PC AT. Because of the broad usage of this bus
structure, it has become known as the Industry Standard
Architecture bus, even though there is no presently accepted
standard for the bus.
- INTERRUPT 13
- This is the software interrupt for disk I/O used by DOS. DOS
does 'Interrupt 13 calls' to read or write from a diskettes. A
SCSI host adapter translates these Interrupt 13 commands into SCSI
commands for SCSI disk drives.
- INTERRUPT 19
- This is the software interrupt that handles the boot function.
The boot code is typically handled by the motherboard BIOS, but
can optionally be handled by the host adapter BIOS with some
Adaptec host adapters.
- I/O
- Refers to an operations, program, or device whose purpose is
to enter data into or to extract data from a computer.
- I/OWARE
- In addition to hardware and software, I/O is the third
critical component for today's PC system performance. Adaptec
I/Oware products speed the flow of data from one part of a system
to another, significantly improving overall performance.
- IRQ
- Interrupt Request Channel. The IRQ of a host adapter can be
changed to several different settings by changing jumpers and/or
switch settings on the adapter board.
- ISA
- Industry Standard Architecture expansion bus. A type of
computer bus used in most PC's. ISA enables expansion devices like
network cards, video adapters and modems to send data to and
receive data from the PC's CPU and memory 16 bits at a time.
Expansion devices are plugged into sockets in the PC's
motherboard. ISA is sometimes called the AT Bus, because it was
originally introduced with the IBM PC-AT in 1983.
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- MCA (Micro Channel Architecture)
- IBM PS/2 (models 50-95) and compatible computers have a MCA
computer bus inside. Can be driven by multiple independent bus
master processors.
- MICROPROCESSOR - BASED ARRAY CONTROLLER
- An array controller in which all management functions are
performed by an on-board microprocessor (for example, the Intel
i960). These arrays offer higher performance than software arrays,
but are relatively expensive. Typically not economically viable
for entry-level and low-end midrange servers.
- MIRRORING
- Also known as RAID 1 or duplexing (when using two host bus
adapters). Full redundancy is obtained by duplicating all data
from a primary disk on a secondary disk. The overhead of requiring
100% data duplication can costly when using more than two drives.
- MTBDL
- Mean time before data loss. The average time before the
failure of a RAID system component causes data to be lost or
corrupted.
- MTBF
- Mean time between failure. Used to measure computer component
average reliability/life expectancy. MTBF is not as well-suited
for measuring the reliability of RAID storage systems as MTBDL,
MTTR or MTDA because it does not account for RAID's ability to
recover from a drive failure. In addition, enhanced enclosure
environments used with RAID systems to increase uptime can further
limit the applicability of MTBF ratings for RAID solutions.
- MULTITASKING
- The execution of commands in such a way that more than one
command is in progress at the same time.
- MULTITHREADING
- A situation in which a host adapter has more than one
outstanding command to two or more SCSI devices.
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- NARROW SCSI DEVICE (as opposed to Wide
SCSI device)
- 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 from 16-bit Wide SCSI devices.
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- 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.
- PARITY PASS THROUGH
- For data integrity an extra bit is available.
- PERIPHERAL
- A SCSI device installed on a computer system.
- PIO (Programmed Input/Output)
- A method of data transfer in which the host microprocessor
transfers data to and from memory via the computer's I/O ports.
PIO enables very fast data transfer rates, especially in
single-tasking operating systems like DOS.
- PLUG AND PLAY (or PLUG & 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.
- PORT I/O ADDRESS
- A window through which software programs communicate commands
to an installed host adapter board. The commands are communicated
8 bits at a time.
- PROGRAMMED INPUT/OUTPUT
- A method of data transfer in which the host microprocessor
transfers data to and from memory via the computer's I/O ports.
PIO enables very fast data transfer rates, especially in
single-tasking operating systems like DOS. The Adaptec AHA-1510
and AHA-1520 host adapters and the AIC-6260 SCSI protocol
controller use this data transfer method.
- PROM
- An acronym for Programmable Read Only Memory. This is a
version of a ROM that is programmable.
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- 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).
- REDUNDANT
- A duplicate disk or component that provides a recovery path in
case of a failure.
- REMOVABILITY
- A feature where the media in a removable media disk drive can
be removed, then replaced with the same or different media without
causing problems to the operating system. If removability was not
supported, media in a removable media drive could not be removed
without potential loss for data unless the computer was turned
off.
- RISC
- A specialized processor that has been designed to handle a
smaller set of instructions for increased performance.
- ROM
- An acronym for Read Only Memory. This is generally a chip on a
computer or I/O card with software programmed inside of it that
controls some function or functions.
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- SCAM (SCSI Configures Auto Magically)
- 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 or 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 (Small Computer Systems Interface)
- 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 DEVICE
- A device such as a host adapter board, fixed disk drive
orCD-ROM drive that conforms to the SCSI interface standard and is
attached to a SCSI bus cable. The device may be an initiator, a
target, or capable of both types of operation.
- SCSI OVERHEAD
- This is the time it takes for the host adapter to internally
process a SCSI command. Adaptec RISC based host adapters have the
advantage of extremely low SCSI overhead, which greatly increases
overall system performance.
- SCSI RESOURCES
- The SCSI specification is available from:
Global Engineering Documents
15 Inverness Way
East
Englewood Co 80112-5704
(800) 854-7179
SCSI-1: X3.131-1986
SCSI-2:
X3.131-199x
SCSI-3: X3T9.2/91-010R4 Working Draft
Global Engineering Documentation in Irvine, CA (714) 261-1455
SCSI-1: Doc # X3.131-1986 from:
ANSI
1430 Broadway
NY, NY 10018
IN-DEPTH EXPLORATION OF SCSI can be obtained
from:
Solution Technology
Attn: SCSI Publications
POB
104
Boulder Creek, CA 95006
(408) 338-4285
(408) 338-4374
FAX
THE SCSI ENCYLOPEDIA and the SCSI BENCH
REFERENCE can be obtained from:
ENDL Publishing
14426 Black Walnut
Ct.
Saratoga, CA 95090
(408) 867-6642
(408) 867-2115 FAX
SCSI: UNDERSTANDING THE SMALL COMPUTER SYSTEM
INTERFACE was published by Prentice-Hall, ISBN
0-13-796855-8
- SINGLE-ENDED SCSI (normal SCSI):
- For each signal that needs to be sent across the bus, there
exists a wire to carry it.
- STANDARD MODE
- The operation mode of the AHA-1740A/1742A/1744 that allows
software drivers written for the AHA-1540/1542/1640 family to
operate fully on the board. This has no performance limitations
but does not allow addressing beyond 16 MBytes in the DOS
environment.
- 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.
Is SYNCHRONOUS faster than
ASYNCHRONOUS?
Asynchronous is faster on short cables,
while synchronous is faster on long cables. The reason has to do
with the propagation delay of the cable; the turn around time of
the silicon; and the interlocked nature of the asynchronous
handshake.
1) We have measured propagation delays from various cables and
found an average of 1.7 nanoseconds per foot, which is roughly
5.25 ns per meter.
2) The turn-around time is the amount of time the SCSI chip
takes to change an output in response to an input. If REQ is an
input then ACK is an output. Or if ACK is an input then REQ is an
output. Typical turn-around time for the 53C90 is 40
nanoseconds.
3) The asynchronous transfer uses an interlocked handshake
where a device cannot do the next thing until it receives positive
acknowledgment that the other device received the last thing.
First REQ goes true (driven by Target), then ACK is
permitted to go true (driven by Initiator), then REQ is
permitted to go false, then ACK is permitted to go false.
Thus we have four "edges" propagating down the cable plus 4
turn-around delays. Asynchronous transfer requires 55 ns setup and
no hold time (paragraph in 5.1.5.1 in SCSI-1 or SCSI-2) which
gives an upper speed limit around 18 MB/s. A detailed analysis
shows that the setup time subtracts out. This is mostly because we
are running at one-third the max rate, but also because setup for
the next byte can begin anytime after ACK is received true or REQ
is received false, depending on who is receiving. You can either
take my word for it or draw the waveforms yourself. Thus, the
asynchronous transfer reduces to:
(4 * 1.7 * 1) + (4 * 40ns) = 167 ns = 6 MB/s (1 foot
cable)
(4 * 5.25 * 6) + (4 * 40ns) = 286 ns = 3.5 MB/s (6 meter
cable)
(4 * 5.25 * 25) + (4 * 40ns) = 685 ns = 1.5 MB/s (25 meter
cable)
Note: cables longer than 6 meters require external differential
transceivers which add delay and degrade the performance even more
than indicated here.
Our simulations say that under very best conditions (fast
silicon, low temperature, high voltage, zero length cable) we can
expect more than 8 MB/s asynchronously. In the lab, I routinely
measure 5 MB/s on 8 foot cables. So, if you were writing the data
manual for this, how would YOU spec it?
The framers of the SCSI spec threw in synchronous mode to boost
the performance on long cables. In synchronous mode, the sending
device is permitted to send the next byte without receiving
acknowledgment that the receiver actually received the last byte.
Kind of a ship and pray method. The acknowledgment is required to
come back sometime, but we just don't have to wait for it
(handwave the offset stuff and the ending boundary conditions). In
this mode any external transceivers add a time shift, but not a
delay. So if you negotiate for 5 MB/s, you get 5MB/s regardless
how long the cable is and regardless whether you are single-ended
or differential. But you can't go faster than 5.5 MB/s, except in
SCSI-2.
Synchronous mode does have a hold time (unlike asynch) but
again, setup and hold times subtract out. In SCSI-1 synchronous
mode, the speed limit comes from the combined ASSERTION PERIOD +
NEGATION PERIOD which is 90ns + 90ns = 180ns = 5.5 MB/s. Our 53C90
family doesn't quite hit the max, but we do guarantee 5.0 MB/s. In
SCSI-2, anything above 5.0 MB/s is considered to be FAST. Here the
maximum transfer rate is explicitly limited to 100 ns or 10MB/s;
you don't have to read between the lines to deduce it.
Interesting tid-bit: given a SCSI-2 FAST period of 100 ns and a
cable delay of 131 ns on a 25 meter cable, you can actually stack
1.31 bytes in the 8-bit cable. In FAST and WIDE SCSI you can stack
5.24 bytes in this copper FIFO.
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- TAGGED QUEUING
- 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)
Without tagged queuing: If tagged queuing was not
enabled, the drive would seek to LBA 0, transfer bytes 0, then 1,
then seek to 101, transfer 101 and 102, then seek back to lba 5,
transfer 5, then 6. ===>involves 3 seeks (initial seek to 5,
seek to 101, then seek back to 5)
With tagged queuing: If tagged queuing was enabled, the
drive would seek to LBA 0, transfer bytes 0, then 1, 5 and 6, then
seek to 101, transferring 101 and 102. At this point all the data
would be transferred. ===>involves 2 seeks (initial seek to 5,
then the seek to 101) Seeking on a disk drive takes a relatively
long time, so having seek and head switches really speeds up
performance.
- 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.
The Single Ended electrical class depends on very tight
termination tolerances, but the passive 132 Ohm termination
defined in 1986 is mismatched with the cable impedance (typically
below 100 Ohms). Although not a problem at low speeds when only a
few devices are connected, reflections can cause errors when
transfer rates increase and/or more devices are added. In SCSI-2,
an active terminator has been defined which lowers termination to
110 Ohms and is a major boost to system integrity.
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- ULTRASCSI
- A method that enables very fast data transfer rate on the SCSI
bus. The maximum UltraSCSI data transfer rates are 20
MBytes/second (40 MBytes/second for Wide SCSI host adapters).
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- VESA (Video Electronics Standards Association)
- This association is responsible for setting standards in all
areas of graphics and video technology.
- VDS (Virtual DMA Services)
- This is a software standard developed by Microsoft so that bus
master host adapters could work efficiently under DOS protected
mode and virtual 86 mode programs. Such programs that use the
protected mode of the processor include MS Windows, Quarterdeck
QEMM and Qualitas 386MAX. The VDS support is either provided in
the DOS device driver (such as in ASPI4DOS.SYS) or by the host
adapter BIOS (such as the AHA-1540CF/1542CF host adapters).
- VL-BUS
- A specification for a local bus developed by the VESA local
bus committee.
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- WARM-SWAP
- The ability to remove and replace a disk drive while the power
is on. All bus activity must be paused to maintain data integrity
during removal or replacement.
- WIDE SCSI
- Provides for performance and compatibility enhancements to
SCSI-1 by adding a 16- or 32- bit data path. Combined with Fast
SCSI, this can result of SCSI bus data transfer rates of 20
MBytes/sec (with a 16-bit bus) or 40 MBytes/sec (with a 32-bit
bus).
SCSI may now transfer data at bus widths of 16 and 32 bits.
Commands, status, messages and arbitration are still 8 bits, and
the B-Cable has 68 pins for data bits. Cabling was a confusing
issue in the closing days of SCSI-2, because the first project of
SCSI-3 was the definition of a 16-bit wide P-Cable which supported
16-bit arbitration as well as 16-bit data transfers. Although
SCSI-2 does not contain a definition of the P-Cable, it is quite
possible that within the year, the P-Cable will be most popular
non-SCSI-2 feature on SCSI-2 products. The market responds to what
it wants, not the the arbitrary cutoffs of standards
committees.
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