5.3. Mass Storage Device Interfaces
Every device used in a computer system must have some means of attaching to that computer system. This attachment point is known as an interface. Mass storage devices are no different — they have interfaces too. Interfaces are important for two main reasons:
There are many different (mostly incompatible) interfaces
Different interfaces have different performance and price characteristics
Unfortunately, there is no single universal device interface and not even a single mass storage device interface. Therefore, system administrators must be aware of the interface(s) supported by their organization's systems. Otherwise, there is a real risk of purchasing the wrong hardware when a system upgrade is planned.
Different interfaces have different performance capabilities, making some interfaces more suitable for certain environments than others. For example, interfaces capable of supporting high-speed devices are more suitable for server environments, while slower interfaces would be sufficient for light desktop usage. Such differences in performance also lead to differences in price, meaning that — as always — you get what you pay for. High-performance computing does not come cheaply.
5.3.1. Historical Background
Over the years there have been many different interfaces created for mass storage devices. Some have fallen by the wayside, and some are still in use today. However, the following list is provided to give an idea of the scope of interface development over the past thirty years and to provide perspective on the interfaces in use today.
- FD-400
An interface originally designed for the original 8-inch floppy drives in the mid-70s. Used a 44-conductor cable with an circuit board edge connector that supplied both power and data.
- SA-400
Another floppy disk drive interface (this time originally developed in the late-70s for the then-new 5.25 inch floppy drive). Used a 34-conductor cable with a standard socket connector. A slightly modified version of this interface is still used today for 5.25 inch floppy and 3.5 inch diskette drives.
- IPI
Standing for Intelligent Peripheral Interface, this interface was used on the 8 and 14-inch disk drives used on minicomputers of the 1970s.
- SMD
A successor to IPI, SMD (stands for Storage Module Device) was used on 8 and 14-inch minicomputer hard drives in the 70s and 80s.
- ST506/412
A hard drive interface dating from the early 80s. Used in many personal computers of the day, this interface used two cables — one 34-conductor and one 20-conductor.
- ESDI
Standing for Enhanced Small Device Interface, this interface was considered a successor to ST506/412 with faster transfer rates and larger supported drive sizes. Dating from the mid-80s, ESDI used the same two-cable connection scheme of its predecessor.
There were also proprietary interfaces from the larger computer vendors of the day (IBM and DEC, primarily). The intent behind the creation of these interfaces was to attempt to protect the extremely lucrative peripherals business for their computers. However, due to their proprietary nature, the devices compatible with these interfaces were more expensive than equivalent non-proprietary devices. Because of this, these interfaces failed to achieve any long-term popularity.
While proprietary interfaces have largely disappeared, and the interfaces described at the start of this section no longer have much (if any) market share, it is important to know about these no-longer-used interfaces, as they prove one point — nothing in the computer industry remains constant for long. Therefore, always be on the lookout for new interface technologies; one day you might find that one of them may prove to be a better match for your needs than the more traditional offerings you current use.
5.3.2. Present-Day Industry-Standard Interfaces
Unlike the proprietary interfaces mentioned in the previous section, some interfaces were more widely adopted, and turned into industry standards. Two interfaces in particular have made this transition and are at the heart of today's storage industry:
SCSI
IDE
5.3.2.1. IDE/ATA
IDE stands for Integrated Drive Electronics. This interface originated in the late 80s, and uses a 40-pin connector.
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Actually, the proper name for this interface is the "AT Attachment" interface (or ATA), but use of the term "IDE" (which actually refers to an ATA-compatible mass storage device) is still used to some extent. The remainder of this section uses the interface's proper name — ATA. |
ATA implements a bus topology, with each bus supporting two mass storage devices. These two devices are known as the master and the slave. These terms are misleading, as it implies some sort of relationship between the devices; that is not the case. The selection of which device is the master and which is the slave is normally selected through the use of jumper blocks on each device.
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A more recent innovation is the introduction of cable select capabilities to ATA. This innovation requires the use of a special cable, an ATA controller, and mass storage devices that support cable select (normally through a "cable select" jumper setting). When properly configured, cable select eliminates the need to change jumpers when moving devices; instead, the device's position on the ATA cable denotes whether it is master or slave. |
A variation of this interface illustrates the unique ways in which technologies can be mixed and also introduces our next industry-standard interface. ATAPI is a variation of the ATA interface and stands for AT Attachment Packet Interface. Used primarily by CD-ROM drives, ATAPI adheres to the electrical and mechanical aspects of the ATA interface but uses the communication protocol from the next interface discussed — SCSI.
5.3.2.2. SCSI
Formally known as the Small Computer System Interface, SCSI as it is known today originated in the early 80s and was declared a standard in 1986. Like ATA, SCSI makes use of a bus topology. However, there the similarities end.
Using a bus topology means that every device on the bus must be uniquely identified somehow. While ATA supports only two different devices for each bus and gives each one a specific name, SCSI does this by assigning each device on a SCSI bus a unique numeric address or SCSI ID. Each device on a SCSI bus must be configured (usually by jumpers or switches[1]) to respond to its SCSI ID.
Before continuing any further in this discussion, it is important to note that the SCSI standard does not represent a single interface, but a family of interfaces. There are several areas in which SCSI varies:
Bus width
Bus speed
Electrical characteristics
The original SCSI standard described a bus topology in which eight lines in the bus were used for data transfer. This meant that the first SCSI devices could transfer data one byte at a time. In later years, the standard was expanded to permit implementations where sixteen lines could be used, doubling the amount of data that devices could transfer. The original "8-bit" SCSI implementations were then referred to as narrow SCSI, while the newer 16-bit implementations were known as wide SCSI.
Originally, the bus speed for SCSI was set to 5MHz, permitting a 5MB/second transfer rate on the original 8-bit SCSI bus. However, subsequent revisions to the standard doubled that speed to 10MHz, resulting in 10MB/second for narrow SCSI and 20MB/second for wide SCSI. As with the bus width, the changes in bus speed received new names, with the 10MHz bus speed being termed fast. Subsequent enhancements pushed bus speeds to ultra (20MHz), fast-40 (40MHz), and fast-80[2]. Further increases in transfer rates lead to several different versions of the ultra160 bus speed.
By combining these terms, various SCSI configurations can be concisely named. For example, "ultra-wide SCSI" refers to a 16-bit SCSI bus running at 20MHz.
The original SCSI standard used single-ended signaling; this is an electrical configuration where only one conductor is used to pass an electrical signal. Later implementations also permitted the use of differential signaling, where two conductors are used to pass a signal. Differential SCSI (which was later renamed to high voltage differential or HVD SCSI) had the benefit of reduced sensitivity to electrical noise and allowed longer cable lengths, but it never became popular in the mainstream computer market. A later implementation, known as low voltage differential (LVD), has finally broken through to the mainstream and is a requirement for the higher bus speeds.
The width of a SCSI bus not only dictates the amount of data that can be transferred with each clock cycle, but it also determines how many devices can be connected to a bus. Regular SCSI supports 8 uniquely-addressed devices, while wide SCSI supports 16. In either case, you must make sure that all devices are set to use a unique SCSI ID. Two devices sharing a single ID will cause problems that could lead to data corruption.
One other thing to keep in mind is that every device on the bus uses an ID. This includes the SCSI controller. Quite often system administrators forget this and unwittingly set a device to use the same SCSI ID as the bus's controller. This also means that, in practice, only 7 (or 15, for wide SCSI) devices may be present on a single bus, as each bus must reserve an ID for the controller.
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Most SCSI implementations include some means of scanning the SCSI bus; this is often used to confirm that all the devices are properly configured. If a bus scan returns the same device for every single SCSI ID, that device has been incorrectly set to the same SCSI ID as the SCSI controller. To resolve the problem, reconfigure the device to use a different (and unique) SCSI ID. |
Because of SCSI's bus-oriented architecture, it is necessary to properly terminate both ends of the bus. Termination is accomplished by placing a load of the correct electrical impedance on each conductor comprising the SCSI bus. Termination is an electrical requirement; without it, the various signals present on the bus would be reflected off the ends of the bus, garbling all communication.
Many (but not all) SCSI devices come with internal terminators that can be enabled or disabled using jumpers or switches. External terminators are also available.
One last thing to keep in mind about SCSI — it is not just an interface standard for mass storage devices. Many other devices (such as scanners, printers, and communications devices) use SCSI. Although these are much less common than SCSI mass storage devices, they do exist. However, it is likely that, with the advent of USB and IEEE-1394 (often called Firewire), these interfaces will be used more for these types of devices in the future.
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The USB and IEEE-1394 interfaces are also starting to make inroads in the mass storage arena; however, no native USB or IEEE-1394 mass-storage devices currently exist. Instead, the present-day offerings are based on ATA or SCSI devices with external conversion circuitry. |
No matter what interface a mass storage device uses, the inner workings of the device has a bearing on its performance. The following section explores this important subject.