Alasir Enterprises
Main Page >  Motley >  MicroHouse PC Hardware Library Volume I: Hard Drives  

 
Previous Table of Contents Next

 
Another advantage is performance. IDE drives are some of the highest-performance drives available — but they also are among the lowest-performance drives. This apparent contradiction is a result of the fact that all IDE drives are different. You cannot make a blanket statement about the performance of IDE drives because each drive is unique. The high-end models, however, offer performance that is equal or superior to that of any other type of drive on the market for a single-user, single-tasking operating system.

IDE Bus Versions

Three main types of IDE interfaces are available, with the differences based on three different bus standards:

  AT Attachment (ATA) IDE (16-bit ISA)
  XT IDE (8-bit ISA)
  MCA IDE (16-bit Micro Channel)

Note:
Many people are confused about 16- versus 32-bit bus connections and 16- versus 32-bit hard drive connections. A PCI connection allows for a 32-bit (and 64-bit in the future) bandwidth from the bus to the drive controller only. In an IDE (or EIDE) drive configuration, you are still getting only 16-bit bandwidth between the drive and the controller. This usually does not create a bottleneck, however, because one or two hard drives cannot supply the controller enough data to saturate even a 16-bit channel. Fast Wide SCSI-3 is the only device/controller combination that gives you 32 bits from the controller to the drive, primarily because you can hang 15 devices off a SCSI Wide chain and there is a good chance that many devices will saturate a 16-bit channel at some time.

The XT and ATA versions have standardized on 40-pin connectors and cables, but the connectors have slightly different pinouts, rendering them incompatible with one another. MCA IDE uses a completely different 72-pin connector and is designed for MCA bus systems only.

In most cases, you must use the type of IDE drive that matches your system bus. This situation means that XT IDE drives work only in XT-class 8-bit ISA slot systems, AT IDE drives work only in AT-class 16-bit ISA or EISA slot systems, and MCA IDE drives work only in Micro-Channel systems (such as the IBM PS/2 Model 50 or higher). A company called Silicon Valley offers adapter cards for XT systems that will run ATA IDE drives. Other companies, such as Arco Electronics and Sigma Data, have IDE adapters for Micro-Channel systems that allow ATA IDE drives to be used on these systems. These adapters are very useful for XT or PS/2 systems, because there is a very limited selection of XT or MCA IDE drives, whereas the selection of ATA drives is virtually unlimited.

In most modern ISA and EISA systems, you will find an ATA connector on the motherboard. If your motherboard does not have one of these connectors and you want to attach an AT IDE drive to your system, you can purchase an adapter card that changes your 98-pin slot connector to the 40-pin IDE connector. These adapter cards are nothing more than buffered cables; they are not really controllers. The controller is built into the drive. Some of the cards offer additional features, such as an on-board ROM BIOS or cache memory.

Because of the limited availability and use of XT IDE and MCA IDE drives, only ATA IDE drive types will be discussed here.

ATA IDE

CDC, Western Digital, and Compaq actually created what could be called the first ATA-type IDE interface drive and were the first to establish the 40-pin IDE connector pinout.

Eventually, the 40-pin IDE connector and drive interface method was placed before one of the ANSI standards committees which, in conjunction with drive manufacturers, ironed out some deficiencies, tied up some loose ends, and published what is known as the CAM ATA (Common Access Method AT Attachment) interface. The CAM Committee was formed in October 1988, and the first working document of the AT Attachment interface was introduced in March 1989. Before the CAM ATA standard, many companies that followed CDC, such as Conner Peripherals, made proprietary changes to what had been done by CDC. As a result, many older ATA drives are very difficult to integrate into a dual-drive setup that has newer drives.

Some areas of the ATA standard have been left open for vendor-specific commands and functions. These vendor-specific commands and functions are the main reason why it is so difficult to low-level format IDE drives. To work properly, the formatter you are using usually must know the specific vendor-unique commands for rewriting sector headers and remapping defects. Unfortunately, these and other specific drive commands differ from OEM to OEM, clouding the “standard” somewhat.


Note:
It is important to note that only the ATA IDE interface has been standardized by the industry. The XT IDE and MCA IDE never were adopted as industry-wide standards and never became very popular. These interfaces no longer are in production, and no new systems of which I am aware come with these nonstandard IDE interfaces.

The ATA Specification

The ATA specification was introduced in March 1989 as an ANSI standard. ATA-1 was finally approved in 1994, and ATA-2 (also called Enhanced IDE) was approved in 1995. ATA-3 is currently in the works. The ATA standards have gone a long way toward eliminating incompatibilities and problems with interfacing IDE drives to ISA and EISA systems. The ATA specifications define the signals on the 40-pin connector, the functions and timings of these signals, cable specifications, and so on. The following section lists some of the elements and functions defined by the ATA specification.

Dual-Drive Configurations

Dual-drive ATA installations can be problematic because each drive has its own controller, and both controllers must function while being connected to the same bus. There has to be a way to ensure that only one of the two controllers will respond to a command at a time.

The ATA standard provides the option of operating on the AT Bus with two drives in a daisy-chained configuration. The primary drive (drive 0) is called the master, and the secondary drive (drive 1) is the slave. You designate a drive as being master or slave by setting a jumper or switch on the drive or by using a special line in the interface called the Cable Select (CSEL) pin.

Previous Table of Contents Next