Parallel ATA
Advanced Technology Attachment (ATA), is a standard interface for connecting storage devices such as hard disk drives and CD-ROM drives inside personal computers. Many terms and synonyms for ATA exist, including abbreviations such as IDE, ATAPI, and UDMA. ATA standards only allow cable lengths in the range of 18 to 36 inches (450 to 900 mm), so the technology normally appears as an internal computer storage interface. It provides the most common and the least expensive interface for this application.
History
Although the standard has always had the official name "ATA", marketing dictates dubbed an early version of the standard Integrated Drive Electronics (IDE), and the one following it Enhanced IDE (EIDE). Although these new names originated in branding convention and not as an official standard, the terms EIDE or E-IDE often appear interchangeably with IDE and ATA. With the introduction of Serial ATA around 2003, this configuration retroactively became renamed as Parallel ATA (P-ATA), referring to the method in which data travels over wires in this interface.
The interface only worked with hard disks at first. Eventually, an extended standard came to work with a variety of other devices -- generally those using removable media. Principally, these devices include CD-ROM drives, tape drives, and large-capacity floppy drives such as the Zip drive and SuperDisk drive. The extension bears the name Advanced Technology Attachment Packet Interface (ATAPI), with the full standard now known as ATA/ATAPI
The movement from programmed input/output (PIO) to direct memory access (DMA) provided another important transition in the history of ATA. Of these methods for accessing and transferring data within computers, PIO proved inefficient, requiring a significant amount of oversight by the computer's CPU. This meant that systems based around ATA devices generally performed disk-related activities much more slowly than computers using SCSI or other interfaces. However, DMA (and later Ultra DMA or UDMA) greatly reduced the amount of processing time the CPU had to use in order to read and write the disks.
ATA devices have suffered from a number of "barriers" in terms of how much data they can handle. However, new addressing systems and programming techniques have broken most of these barriers. Some of the ATA-specific barriers included: 504 MB, 32 GB, and 137 GB. A variety of other barriers have existed, usually due to poorly-written drivers and disk input/output layers in operating systems. Even the barriers listed above mostly came about due to poor BIOS implementations.
Parallel ATA Interface
Until the introduction of Serial ATA, 40-pin connectors generally attached drives to a ribbon cable. Each cable has two or three connectors, one of which plugs into a controller that interfaces with the rest of the computer system. The remaining one or two connectors plug into drives. Parallel ATA cables transfer data 16 or 32 bits at a time. One occasionally finds cables that allow for the connection of three ATA devices onto one IDE channel, but in this case one drive remains read-only (this type of configuration virtually never occurs).
For most of ATA's history, ribbon cables had 40 wires, but an 80-wire version appeared with the introduction of the Ultra DMA/66 standard. The 80-wire cable provides one ground wire to each signal wire. This reduces the effects of electromagnetic induction between neighboring wires and enables the 66 megabyte per second (MB/s) transfer rate of UDMA4. The faster UDMA5 and UDMA6 standards require 80-conductor cables. Though the number of wires doubled, the number of connector pins remains the same as on 40-conductor cables. The physical connectors are identical between the two cable types.
With the release of successive high-speed versions of the ATA standard in the late 1990s, the original cable length specifications became much less generous. With a maximum length of just 18 inches (450 mm), difficulties can ensue in connecting drives within large computer cases, or when mounting several physical drives into one computer.
If two drives attach to a single cable, the configuration generally sees one as a master and the other as a slave. The master drive generally shows up ahead of the slave drive when the computer's operating system enumerates available drives. The master drive arbitrates access to devices on the channel. Because of this, latency-sensitive devices such as early CD-RW drives often benefitted from functioning as a master, and each channel must have a master in order to function properly.
Most systems configure a single drive as a master. However, some drives have a special setting called single for this mode of operation (the brand Western Digital, in particular, utilizes this additional setting). Also, depending on the hardware and software available, a single drive can operate as the slave drive.
A drive setting called cable select has also emerged. In this mode of operation, the drives automatically configure themselves as master or slave. This is achieved by cutting wire 28 (on 40 wire cables, or wires 56 and 57 on 80 wire cables) between the two HDD/CDROM connectors. Some newer cables have this done internally in the connectors. In this case, the two connectors are of different colours.
| Mode | Speed |
|---|---|
| UDMA0 | 16.7 MB/s |
| UDMA1 | 25.0 MB/s |
| UDMA2 | 33.3 MB/s |
| UDMA3 | 44.4 MB/s |
| UDMA4 | 66.7 MB/s |
| UDMA5 | 100.0 MB/s |
| UDMA6 | 133 MB/s |
Note that the transfer rate for each UDMA mode gives its maximum theoretical transfer rate. Protocol overhead effectively reduces this value, and other factors such as PCI or interconnect bus congestion may reduce transfer rates even further. In addition, as of February 2004 no IDE hard drives exist capable of sustaining transfers at or above 50 MB/s, so these transfer limits only really affect performance when the hard drive operates in burst mode, which means that the requested data comes from its cache and the drive therefore does not have to read the data from its platters.
Serial ATA
The largest change in ATA happened with the introduction of Serial ATA. This interface uses 7-pin cables for the data connection, and transmits the data serially rather than in parallel. In addition, Serial ATA should give users the ability to hot swap hard drives. This adds a capability that more expensive systems such as SCSI and Fibre Channel have had for a long time, though the future will tell how widely users exploit that aspect of the technology. Serial ATA also reduces the signalling voltage from the 5 volts used in P-ATA down to 0.5 volts, which reduces power consumption and electrical interference. Due to serial transfer and lower power the maximum allowable length of SATA cables exceeds that of ATA ribbon cables, which eliminates some of the problems mentioned previously.
The transition to Serial ATA should largely remain transparent to operating systems, though these will probably need to add new features to make full use of the technology.