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 | | | | Aug31Written by:D. Glen Cardenas
Thursday, August 31, 2000 6:00 PM  IDE, or more formally, IDE/ATA, is the most common system for connecting a hard drive to a PC.
In modern systems (to which this discussion is limited), they plug directly into the motherboard through a 40 pin cable. Most motherboards offer 2 separate IDE channels and thus 2 connectors on the board. Each connector can support 2 IDE devices, be they disk drives, CD drives, tape drives, removable drives and so on. If a channel has 2 devices on it, one must be designated a master and the other a slave. This is done simply by moving or removing a jumper on the drive itself.
As a result of this configuration, any system can have 4 IDE devices connected to it. Using an external controller board connected to the PCI bus supporting 2 additional channels, up to 8 devices and be supported on a PC. This is the limit, and attempting to add 4 more devices with an extra controller will consume more interrupts and other system resources. This contrasts with modern SCSI which can have up to 15 devices on a controller and occupies the same amount of system resources regardless of the number of devices connected up to that limit.
The History of IDE
IDE replaces older interfaces such as ST-506 and ESDI. Through the years, many changes have been made to the IDE standard as defined by ANSI.
The original standard, call simply ATA called for 2 devices on the same channel configured as master and slave. It also defined PIO modes 0, 1 and 2 and DMA single word modes 0, 1 and 2 and multiword mode 0. However, this standard had problems. Often drives by different manufacturers wouldn't work if combined on a single channel as master and slave. ATA-2 added the faster PIO modes 3 and 4 (mode 4 being the common default PIO mode for modern PCs), faster DMA multiword modes 1 and 2, the ability to do block mode transfers, Logical Block Addressing or LBA, and improved support for the "identify drive" command that allows the system to interrogate the drive for manufacturer, model and geometry.
The terms "Fast ATA and Fast ATA-2" are the inventions of Seagate and Quantum. They are not really standards and only denote drives that are compliant to all or part of the ATA-2 standard. ATA-3, however, was a real standard that improved reliability and defined the SMART feature in disk drives. It was followed by the current Ultra ATA or UATA. UATA also goes by many other names like UDMA, DMA-33/66 and ATA-33/66.
UATA isn't really a new standard, and UATA drives are still backward compatible with ATA and ATA-2 systems. Ultra ATA is the term given to drives that support the new DMA modes that provide up to 33 MB/s (UDAM-33) or up to 66 MB/s (UDMA-66) transfer rates with 100 MB/s just over the next hill. Both UDMA versions support CRC error checking that assures data integrity through the IDE cable, which was a source of serious problems in previous standards. Note that the UDMA-66 standard calls for an 80 conductor cable instead of the 40 conductor cable used up to and through UDMA-33.
EIDE or Enhanced IDE is a designation created by Western Digital to describe its newer line of high speed drives. It really isn't a standard at all, but just a marketing tool. However, it has taken on common public use to refer to all high speed drives and the systems that support them.
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