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Chapter 2
Hard Disk Interfaces

This chapter describes the hard disk interface, from the drives to the cables and controllers that run them. You learn about the various disk interfaces you can select, and the shortcomings and strengths of each type.

Interfaces Choices

The primary job of the hard disk controller or interface is to transmit and receive data to and from the drive.

Several types of hard disk interfaces have been used in PC systems over the years:

• ST-506/412 • IDE

Of these interfaces, only ST-506/412 and ESDI are what you could call true disk-controller-to-drive interfaces. SCSI and IDE are system-level interfaces that usually incorporate a chipset-based variation of one of the other two types of disk controller interfaces internally. For example, most SCSI and IDE drives incorporate the same basic controller circuitry used in separate ESDI controllers. The SCSI interface adds another layer of interface that attaches the controller to the system bus, whereas IDE is a direct bus-attachment interface.

In data recovery, it helps to know the disk interface you are working with, because many data-recovery problems involve drive setup and installation problems. Each interface requires a slightly different method of installation and drive configuration. If the installation or configuration is incorrect or accidentally altered by the system user, it may prevent access to data on a drive. Accordingly, anyone who wants to become proficient in data recovery must be an expert on installing and configuring various types of hard disks and controllers.

The ST-506/412 Interface

The ST-506/412 interface was developed by Seagate Technologies around 1980. Most drive manufacturers that made hard disks for PC systems adopted the Seagate ST-506/412 standard, a situation that helped make this interface popular.

The ST-506/412 interface does not quite make the grade in today's high-performance PC systems. This interface was designed for a 5MB drive, and I have not seen any drives larger than 152MB (Modified Frequency Modulation encoding) or 233MB (Run-Length Limited encoding) available for this type of interface. Because the capacity, performance, and expandability of ST-506/412s are so limited, this interface is obsolete and generally unavailable in new systems. However, many older systems still use drives that have this interface.

The ESDI Interface

ESDI, or Enhanced Small Device Interface, is a specialized hard disk interface established as a standard in 1983, primarily by Maxtor Corporation.

Compared with ST-506/412, ESDI has provisions for increased reliability, such as building the endec into the drive. ESDI is a very-high-speed interface, capable of a maximum 24Mbps transfer rate. Most drives running ESDI, however, are limited to a maximum 10Mbps or 15Mbps. Unfortunately, compatibility problems between different ESDI implementations, combined with pressure from low-cost, high-performance IDE interface drives, have served to make the ESDI interface obsolete. Few, if any, new systems today include ESDI drives, although ESDI became somewhat popular in high-end systems during the late 1980s.

I no longer recommend installing ESDI drives unless you are upgrading a system that already has an ESDI controller.

The IDE Interface

Integrated Drive Electronics (IDE) is a generic term applied to any drive with an integrated (built-in) disk controller. The IDE interface as we know it is officially called ATA (AT Attachment) and is an ANSI standard; however, IDE can roughly apply to any disk drive with a built-in controller.

In a drive with IDE, the disk controller is integrated into the drive, and this combination drive/controller assembly usually plugs into a bus connector on the motherboard or bus adapter card. Combining the drive and controller greatly simplifies installation, because there are no separate power or signal cables from the controller to the drive. Also, when the controller and the drive are assembled as a unit, the number of total components is reduced, signal paths are shorter, and the electrical connections are more noise-resistant, resulting in a more reliable design than is possible when a separate controller, connected to the drive by cables, is used.

Integrating the controller and drive also frees the controller and drive engineers from having to adhere to the strict standards imposed by the earlier interface standards. The resulting drive and controller combinations can offer higher performance than earlier stand-alone controller and drive setups. IDE drives sometimes are called drives with embedded controllers.

The IDE connector on motherboards in many systems is nothing more than a stripped-down bus slot. In ATA IDE installations, these connectors normally contain a 40-pin subset of the 98 pins that would be available in a standard 16-bit ISA bus slot. The pins used are only the signal pins required by a standard-type XT or AT hard disk controller. For example, because an AT-style disk controller uses only interrupt line 14, the motherboard AT IDE connector supplies only that interrupt line. No other interrupt lines are needed. The XT IDE motherboard connector supplies interrupt line 5 because that is what an XT controller would use.

When IDE drives are discussed, the ATA IDE variety usually is the only kind mentioned because it is so popular. But other forms of IDE drives exist, based on other buses. For example, several PS/2 systems came with Micro-Channel (MCA) IDE drives which plug directly into a Micro-Channel Bus slot (through an angle adapter or Interposer card). An 8-bit ISA form of IDE also existed but was never very popular. Most IBM-compatible systems with the ISA or EISA Bus use AT-Bus (16-bit) IDE drives. The ATA IDE interface is by far the most popular type of drive interface available.

The primary advantage of IDE drives is cost. Because the separate controller or host adapter is eliminated and the cable connections are simplified, IDE drives cost much less than a standard controller-and-drive combination. These drives also are more reliable, because the controller is built into the drive. Therefore, the endec or data separator (the converter between the digital and analog signals on the drive) stays close to the media. Because the drive has a short analog-signal path, it is less susceptible to external noise and interference.

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