Glossary - Computer Hard Drives

IDE / EIDE / ATA / ATA-2 / ATA66 / ATAPI / Ultra DMA / Ultra DMA/66 / Ultra DMA Q&A / SCSI / SCSI-1 / SCSI-2 / SCSI-3 / Single-Ended and Differential / Narrow and Wide / Fast and Ultra / Serial SCSI (FireWire) / Links

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IDE Interface
Abbreviation of either Intelligent Drive Electronics or Integrated Drive Electronics, depending on who you ask. An IDE interface is an interface for mass storage devices, in which the controller is integrated into the disk or CD-ROM drive.
Although it really refers to a general technology, most people use the term to refer the ATA specification, which uses this technology. Refer to ATA for more information.

EIDE
Short for Enhanced IDE, a newer version of the IDE mass storage device interface standard developed by Western Digital Corporation. It supports data rates of between 4 and 16.6 MBps, about three to four times faster than the old IDE standard. In addition, it can support mass storage devices of up to 8.4 gigabytes, whereas the old standard was limited to 528 MB. Because of its lower cost, enhanced EIDE has replaced SCSI in many areas.

EIDE is sometimes referred to as Fast ATA or Fast IDE, which is essentially the same standard, developed and promoted by Seagate Technologies. It is also sometimes called ATA-2.

There are four EIDE modes defined. The most common is Mode 4, which supports transfer rates of 16.6 MBps. There is also a new mode, called ATA-3 or Ultra ATA, that supports transfer rates of 33 MBps.

ATA
Short for AT Attachment, a disk drive implementation that integrates the controller on the disk drive itself. There are several versions of ATA, all developed by the Small Form Factor (SFF) Committee:
?amp;nbsp; ATA: Known also as IDE, supports one or two hard drives, a 16-bit interface and PIO modes 0, 1 and 2.
?amp;nbsp; ATA-2: Supports faster PIO modes (3 and 4) and multiword DMA modes (1 and 2). Also supports logical block addressing (LBA) and block transfers. ATA-2 is marketed as Fast ATA and Enhanced IDE (EIDE). ATA-2 has an extension called ATAPI.
?amp;nbsp; ATA-3: Minor revision to ATA-2.
?amp;nbsp;Ultra-ATA: Also called Ultra-DMA, ATA-33, and DMA-33, supports multiword DMA mode 3 running at 33 MBps.
?amp;nbsp; ATA/66: A new version of ATA proposed by Quantum Corporation, and supported by Intel, that will double ATA's throughput to 66 MBps. The first ATA/66 computers are expected to be available in the first half of 1999.

ATAPI
Short for AT Attachment Packet Interface, an extension to EIDE (also called ATA-2) that enables the interface to support CD-ROM players and tape drives

Ultra DMA
A protocol developed by Quantum Corporation and Intel that supports burst mode data transfer rates of 33.3 MBps. This is twice as fast as the previous disk drive standard for PCs, and is necessary to take advantage of new, faster Ultra ATA disk drives.
The official name for the protocol is Ultra DMA/33. It's also called UDMA, UDMA/33 and DMA mode 33. See also Q&A.

Ultra DMA/66
A new version of ATA proposed by Quantum Corporation, and supported by Intel, that will double ATA's throughput to 66 MBps. It’s also called ATA/66.

Ultra DMA Q&A
Q01: What is Ultra DMA?
Q02: Does Ultra DMA provide any other advantage?
Q03: How does Ultra DMA compare with Ultra SCSI controller?
Q04: Are there any special system or software requirements to use an Ultra DMA drive?
Q05: Are Ultra DMA drives backward compatible with older systems?
Q06: How can I tell if my motherboard supports Ultra DMA?
Q07: Is there any way of obtaining Ultra DMA capability without a motherboard with the aforementioned chipsets?
Q08: What is burst mode?

• Q01: What is Ultra DMA?

A01: Ultra DMA (UDMA) is the latest advancement to the ANSI ATA specifications. For detailed information about the ANSI ATA specifications view them at the T13 Committee site at ftp://fission.dt.wdc.com/pub/standards/x3t13/t13.htm. ATA-4, among other improvements, supports Ultra DMA modes 0, 1 and 2. UDMA mode 2 supports burst data transfer rates up to 33 MB per second (MB/s). ATA-5, among other improvements, supports Ultra DMA modes 3 and 4. UDMA mode 4 supports burst data transfer rates up to 66 MB/s. ATA-5 Extensions Synchronous DMA Mode for Ultra DMA  

* - Equivalent to PIO mode 4 (ATA-3)
 
UDMA/33 (ATA-4) doubles and UDMA/66 (ATA-5) quadruples the maximum transfer speed of the ATA-3 interface while maintaining the cycle time of the ATA bus clock at the rate used by PIO Mode 4.

This apparent miracle is achieved by:

Having the entity transmitting the data, either host or device, provide the clocking signal for the data
Using both edges of the clocking signal to strobe the data
For modes 3 and 4, improving the cable connecting the devices to the host.

Top of Q&A

• Q02: Does Ultra DMA provide any other advantages?

A02: In addition to increasing throughput, Ultra DMA/33 improves data integrity by using a Cyclic Redundancy Check (CRC) to flag any data transfer errors that may be made over the ATA bus.
NOTE: In this application, CRC is only used to improve data integrity for ATA bus transfers, it is not used to improve the data integrity of either disk drives or host systems. All Maxtor hard drives include a powerful proprietary ECC (Error Correction Code) to insure data integrity when writing to or reading from the drive.

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• Q03: How does Ultra DMA compare with Ultra SCSI controllers?

A03: Ultra DMA has tested faster than Ultra Wide SCSI under WinMark97. User's can anticipate the high performance of Ultra DMA at half the price of SCSI.

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• Q04: Are there any special system or software requirements to use an Ultra DMA drive?

A04: Yes as follows:
To utilize the UDMA capability of a hard drive, the system must have the appropriate chip sets and software drivers. The motherboard must be designed with the interface chip sets that provide the UDMA support and the motherboard manufacturer must provide the drivers necessary to implement the UDMA operations.
Motherboard manufacturers of the current PII systems provide the necessary support for UDMA/33. Older Pentium based systems with the TX; LX and BX chip sets should also provide support for UDMA/33.
As of the release of this Q&A the chip sets that support UDMA/66 to be used by motherboard manufacturers have not been identified. However a chip set providing UDMA/66 and the UDMA/66 software drivers are required in order to obtain UDMA/66 performance.

Operating Systems (OS) do not normally include the UDMA drivers, as the OS does not have any direct control over this operation. If the motherboard provides the chip sets and the drivers are installed, after the OS, then the system will automatically utilize the maximum data transfer rate possible when transferring data between RAM and the hard drive.

For systems which have all of the pre-requisites for UDMA mode 3 and 4 and a hard drive capable of UDMA modes 3 and 4 a special 80 wire, 40-pin interface cable is required. The 80 wire cable reduces crosstalk and improves signal integrity by providing 40 additional ground lines between the 40-pin IDE signal and ground lines. Due to the higher transfer rate and shorter cycle times of the data transfers this cable is required to achieve UDMA mode 3 and 4 performance.

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• Q05: Are Ultra DMA drives backward compatible with older systems?

A05: The short answer is yes, however there are issues to be aware of. Any EIDE drive can be used on an older system. The hard drives performance is reduced to the maximum capability of the system it is installed in.
If a UDMA/66 drive is installed in a system with a maximum capability of UDMA/33 then the UDMA/66 drive is limited to UDMA/33. If a UDMA/66 or UDMA/33 drive is installed in a system with a maximum performance of PIO Mode 4 then the drives are limited to the system PIO Mode 4 performance. The older the system the more limited is its performance. In older systems where UDMA/66 performance cannot be achieved the use of the 80-wire UDMA/66 IDE interface cable is not required.

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• Q06: How can I tell if my motherboard supports Ultra DMA?

A06: Pentium motherboards that have the Intel TX Chipset, Pentium II motherboards starting with the Intel LX, BX and subsequent Intel Chipsets have UDMA capability. The chipset is displayed on system startup. If unsure, consult system or motherboard documentation or manufacturer for chipset and UDMA performance verification.

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• Q07: Is there any way of obtaining Ultra DMA capability without a motherboard with the aforementioned chipsets?

A07: The answer is yes. This can be achieved by the purchase and installation of an UDMA EIDE Interface (I/F) Card. As of the publication of this Q&A there two known manufacturers of UDMA I/F cards that support UDMA/33, check with them regarding their plans to produce a UDMA/66 capable card at the following Websites:
Promise Technology: http://www.promise.com/
SIIG: http://www.siig.com/

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• Q08: What is burst mode?

A08: A data transmission mode in which data is sent faster than normal. There are a number of techniques for implementing burst modes. In a data bus, for example, a burst mode is usually implemented by allowing a device to seize control of the bus and not permitting other devices to interrupt. In RAM, burst modes are implemented by automatically fetching the next memory contents before they are requested. This is essentially the same technique used by disk caches.

The one characteristic that all burst modes have in common is that they are temporary and unsustainable. They allow faster data transfer rates than normal, but only for a limited period of time and only under special conditions.

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Small Computer Systems Interface (SCSI)
The second-most popular hard disk interface used in PCs today is the Small Computer Systems Interface, abbreviated SCSI and pronounced "skuzzy". SCSI is a much more advanced interface than its chief competitor, IDE/ATA, and has several advantages over IDE that make it preferable for many situations, usually in higher-end machines. It is far less commonly used than IDE/ATA due to its higher cost and the fact that its advantages are not useful for the typical home or business desktop user.
In terms of standards, SCSI suffers from the same problem  that IDE/ATA does: there are too many different ones and it can be hard to understand what is what. Fortunately, this situation is coming under control now. Also, SCSI standards aren't as much of a problem as they are for IDE/ATA, because in the SCSI world, each SCSI protocol has a name that indicates rather clearly what its capabilities are, and there is much less reliance on using the name of the standard to infer transfer rates and other characteristics. For example, in the IDE/ATA world you will often hear a drive called "EIDE", and then have to figure out what this means. In the SCSI world it is more typical for drives to be called "Fast Wide SCSI" or "Ultra SCSI", which tells you the basic transfer characteristics of the interface, instead of relying on the name of a particular standard. Unfortunately, there is still a lot of confusion if you try to figure out the standards themselves and what each one means. And there are also many manufacturers playing fast and loose with how they label their drives.
SCSI is a much higher-level protocol than IDE is. In fact, while IDE is an interface, SCSI is really a system-level bus, with intelligent controllers on each SCSI device working together to manage the flow of information on the channel. SCSI supports many different types of devices, and is not at all tied to hard disks the way IDE/ATA is (ATAPI supports non-hard-disk IDE devices but it is really a kludge of sorts). Since it has been designed from the ground up as almost an additional bus for peripherals, SCSI offers performance, expandability and compatibility unmatched by any other current PC interface.
Despite the fact that IDE stands for "integrated drive electronics", and that IDE and SCSI are "competing" interfaces, SCSI devices all have integrated drive controllers. IDE is really a misnomer for the IDE/ATA interface.
There are many different flavors of SCSI, which run at different bit widths and speeds. Unlike the world of IDE/ATA, where there are many different competing standards that in many cases are just competing marketing terms put forth by manufacturers, SCSI standards are relatively, well, standard. It has taken some time for the SCSI standards to "settle down" and become widely adopted. In its early days, the lack of widely-accepted standards probably hindered the acceptance of SCSI in the PC marketplace.
SCSI standards are defined by ANSI, and define characteristics and capabilities of the interface. Particular SCSI implementations are normally referred to by the transfer protocol (width and speed) they use, and not by a SCSI standard. In fact, the two main standards, SCSI-1 and SCSI-2, are basically compatible.
One problem with these standards is that it is hard in many cases to draw the line between them. There are features that seem to be part of SCSI-2 that are really SCSI-3, for example. As always, the best policy is to make absolutely sure that you know what you are getting, when you go to purchase.

SCSI-1
The original SCSI standard was approved by ANSI in 1986 as standard X3.131-1986. It defines the basics of the first SCSI buses, including cable length, signaling characteristics, commands and transfer modes.
Original SCSI was far more limited than its successor, SCSI-2. It defined only the most basic 8-bit narrow bus, and 5 MB/s transfer rate. It also had difficulties in terms of gaining universal acceptance due to the fact that many manufacturers implemented different subsets of its features. It was replaced gradually by SCSI-2.
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.
SCSI-2

SCSI-2
The advanced SCSI-2 specification was approved by ANSI in 1990. It is an extensive enhancement of the original standard, and defines support for many of the more advanced SCSI features that are in wide use today.
SCSI-2 defines the following significant new features as enhancements to the original SCSI-1 specification:
Fast SCSI: This high-speed transfer protocol doubles the speed of the bus to 10 MHz, meaning 10 MB/s transfer rate with 8-bit regular SCSI cabling or even higher when used with Wide SCSI.
Wide SCSI: Widening the original 8-bit SCSI bus to 16 bits or even 32 bits permits more data throughput at a given signaling speed.
More Devices per Bus: On buses that are running with Wide SCSI, 16 devices are supported (as opposed to 8 with regular SCSI).
Improved Cables and Connectors: As discussed in detail here, SCSI uses a confusingly large number of different cable and connectors. SCSI-2 defined new higher-density connections.
Active Termination: Termination is an important technical consideration in setting up a SCSI bus. SCSI-2 defined the use of active termination, which provides more reliable termination of the bus.
Command Queuing: One of SCSI's strengths is its ability to allow multiple outstanding requests between devices on the bus, simultaneously. This was introduced in SCSI-2.
Additional Command Sets: SCSI-2 added new command sets to support the use of more devices such as CD-ROMs, scanners and removable media. The older command set focused more on hard disks.
Command Set Enhancements: The command sets for controlling all kinds of devices were enhanced, including diagnostic capabilities and messaging.
It is important to note that one of the major design criteria in the creation of SCSI-2 was backward compatibility with SCSI-1. SCSI-2 devices will in most cases work with older SCSI-1 devices on a bus, however this is not always done, because the older devices have no ability to support the SCSI-2 enhancements and faster transfer protocols.

SCSI-3
SCSI-3 is kind of confusing. It seems that there are a number of different possible features that are vying for inclusion in this standard, which is still not formalized. Some of these conflict with each other or represent totally different approaches to how SCSI is to be implemented or used. SCSI-3 also seems to include all of what SCSI-2 included, so there sometimes appears to be significant overlap.
The following are the features commonly referred to as being part of SCSI-3:
Ultra SCSI: A further doubling of system bus speed, defining transfer rates up to 20 MHz, meaning 20 MB/s with 8-bit SCSI, or more with Wide SCSI.
Improved Cabling: SCSI-3 again improves cabling over the improvements in SCSI-2, for the use of Wide SCSI.
Serial SCSI (Firewire): SCSI-3 contains as one of its different protocol standards the description for the new Serial SCSI, also called Firewire.

Single-Ended and Differential SCSI
SCSI is a high-speed bus capable of supporting multiple devices, including devices connected to the outside of the PC. Due to the high speed, and the external cabling in particular, there is always concern about signal integrity on the bus. The longer the cables are, the more problems there potentially can be with signal degradation or interference. The faster the bus runs, the more difficult it is to keep the signals clean.
SCSI has therefore defined two different electrical signaling systems:
Single-Ended SCSI: This is "regular" SCSI, and uses the type of conventional signaling that is used on other buses. Basically, a positive voltage is a "one", and ground (zero voltage) is a "zero", and each signal is carried on one wire. This is by far the most common type of SCSI, and therefore offers the most flexibility and the most cost-effective solutions. However, the cable length of the bus is extremely limited.
Differential SCSI: This form of SCSI uses a form of differential signaling, where each signal is actually carried by two different wires, each the mirror image of the other. So here, a "one" is represented by a positive voltage on one wire, and an equal but opposite negative voltage on another wire; a "zero" is electrical ground or zero voltage on both wires. This use of two conductors per signal makes the signal much more resilient and less likely to be corrupted. This allows the use of much longer cabling than single-ended SCSI, but the cost is much higher.
The various transfer rate protocols are defined for potential use in each of these electrical flavors. So you can have single-ended Fast Wide SCSI, or differential Fast Wide SCSI. (This doesn't necessarily mean that all of the different protocols are readily available in both single-ended and differential). Overall, differential SCSI is used far less and is much more expensive. It is not often encountered in the PC world.
Warning: Single-ended and differential SCSI are incompatible at the electrical level. You should not mix single-ended and differential SCSI devices on the same bus or actual physical damage could result. To compound the matter, the cables used for single-ended and differential SCSI look the same. Make sure you know what you have before putting together your SCSI bus. Converters between single-ended and differential SCSI are available.

SCSI Bus Width (Narrow and Wide)
There are two commonly used SCSI bus widths: narrow and wide. Narrow SCSI uses a data pathway that is 8 bits wide. Wide SCSI uses a data pathway 16 bits wide. Narrow SCSI is "conventional" and is what the original forms of SCSI used. Wide SCSI is newer and allows for doubling of bus bandwidth, at a higher cost. It also requires either additional or newer cabling. Wide SCSI also allows the use of 16 devices on the SCSI bus, as opposed to only 8 for regular "narrow" SCSI.
Regarding terminology, the narrow SCSI bus is considered the "regular" or default type, so it is not usually mentioned in the name of the protocol. Wide SCSI has the name "wide" inserted in the protocol name. So for example, "Fast SCSI" implies a narrow bus, while "Fast Wide SCSI" of course is wide.
It is possible to mix narrow and wide SCSI on the same bus, but there are problems that must be overcome to do so. These typically revolve around cabling, which is different for narrow and wide SCSI, and also with termination. Adapters are generally required to convert between the narrow and wide cables.
Note: A "very wide" form of SCSI that is actually 32 bits wide was defined as part of the SCSI-2 standard but has not been popularly implemented and is not generally encountered in the PC world.

SCSI Bus Speed (Regular, Fast and Ultra)
There are three different bus speeds used in SCSI today:
Regular: The default speed for SCSI is 5 MHz. This is the bus defined in the original SCSI-1 specification. Buses running at regular speed have a transfer rate of 5 MB/s for narrow SCSI, or 10 MB/s for wide SCSI.
Fast: Fast SCSI increases bus speed to 10 MHz. The doubling of this theoretical transfer rate was defined as part of SCSI-2. Buses running at this speed have a transfer rate of 10 MB/s for narrow SCSI, or 20 MB/s for wide SCSI.
Ultra (Fast-20): The SCSI-3 specification defines timing that again doubles the interface transfer rate, to 20 MHz (which is why it is also sometimes called Fast-20). Ultra SCSI buses have a maximum transfer rate of 20 MB/s for narrow SCSI, or 40 MB/s for wide SCSI.
Faster bus speeds of course offer more performance. They are also usually more expensive and generally have more stringent cable length restrictions and termination requirements. Remember that these transfer rates are theoretical rates for the interface. No individual device will generate enough sustained data to saturate a 20 MB/s interface, and the numbers ignore command overhead and other subtleties that always lower the maximum effective throughput. However, since SCSI can support many devices, the high-speed interfaces can be useful in multitasking environments where many devices can be talking to each other simultaneously.

Serial SCSI / FireWire
All of the "conventional" types of SCSI that have been used since the interface was created, have been forms of what is called parallel SCSI. This term refers to the fact that the data is transmitted 8 or 16 bits at a time, in parallel. A new type of SCSI, called Serial SCSI, takes a different approach to the SCSI bus by transmitting just one bit at a time. The distinction between parallel and serial here is very similar to the difference between the serial and parallel ports at the back of your PC, which you probably use for your mouse and printer or other devices.
On the surface, going from 8 or 16 bits of data being transmitted at a time to one, might seem like a bit step backwards. The bandwidth of a bus is directly proportional to its width; why reduce it by a factor of 16? The answer is the other factor that controls bus performance: speed.
As technology improves, our appetite for bandwidth continues to increase, and the desire to increase bus speeds has led us from regular to Fast to Ultra SCSI. The problem is that each time the bus is made faster, it gets more difficult to manage the complex signaling on the parallel SCSI bus, and to ensure that there is no data corruption on the cable due to interference or signal degradation. This is why the maximum cable length for single-ended SCSI decreases by half each time the speed doubles.
The 20 MHz of Ultra SCSI is close to the top end of what is achievable using the old style bus. Serial SCSI, which also goes by the nickname Firewire, trades in the width of the original SCSI bus in favor of dramatic increases in speed. Since only a single data line must be managed, it is possible to increase its speed from the 20 MHz maximum of Ultra SCSI, to 400 MHz or even higher. Ultimately speeds of over 1 GHz will be possible; even if you divide this by 16 you get 64 MB/s, which is much higher than Ultra SCSI's 40 MB/s.
Furthermore, the serial connection is much simpler than the large, cumbersome SCSI connections of old. Instead of a 68-wire cable, Firewire uses a 6-wire cable. The serious concerns about termination and signal delay are also addressed. Serial SCSI devices promise to have even more widespread support than older SCSI did. In addition to the PC platform, it will be supported by Apple, and perhaps more interestingly, by non-computer hardware as well. In fact, one of the first types of Firewire devices were digital video cameras, using Firewire to connect to the PC.
Firewire has been formalized as IEEE standard 1394. A trade association has been formed to further the advancement of the standard.
 

Part of the above informations are from following hot links:
The PC Guide
PC Webopedia