Parallel and Serial ATA

The Parallel Interface

Parallel ATA or IDE has been the dominant PC interface protocol for desktop PCs since the 1980s, because of its simplicity, good performance, and low cost. Parallel ATA has been enhanced over the years, data transfer speed improving from 3 MB/s to 133 MB/s (that’s 1 Gb/s). Other improvements include

• Enhanced Integrated Drive Electronics (EIDE) extensions for faster hard-drive access and logical block addressing (LBA)
• ATA Packet Interface (ATAPI) for support of other peripheral devices, such as CD-ROM, DVD-ROM,CD-RW, and DVD+RW drives
• Multiple data-transfer modes, including Programmed Input/Output (PIO), direct memory access (DMA), and Ultra DMA (UDMA)
• Some Cyclic redundancy checking (CRC) for improved data protection and greater overall data integrity

However Parallel ATA has a number of limitations that make further development difficult

High 5-Volt Signaling

Parallel ATA uses 5-volt signaling, which does not match modern low voltage chips. Each ATA channel needs 26 * 5-volt signals, and large physical chip pads to accommodate the high pin count. The chip pads are now almost larger than the chips. The decline in chip sizes means the smaller silicon geometries require lower voltages. This makes the 5V parallel ATA requirement difficult to engineer.

Cable Issues

A parallel ATA cable is a wide, flat 18-inch ribbon. It is difficult to route this cable inside a chassis, and its bulky size restricts the cooling air flow, which creates hot spots. This is especially problematic with today's increasing processor and graphics subsystem power requirements. The fact that the cable length is limited to 18 inches limits the choices for adding peripherals and internal drives.

Data Integrity

It is necessary to use some sort of redundant or parity information to check that data is being transmitted properly. Early ATA disks did not check the data at all. Cyclic Redundancy Checking (CDC) was introduced with UDMA, which verifies the interface data, but not the command data. This remains a potential error source.

The Serial ATA Solution

Traditionally, serial communications were regarded as slow, but new standards such as USB 2.0, Firewire, Ethernet, V-Link, MuTIOL, HyperTransport, RapidIO are all serial-based, and are fast.
Version 1 of Serial ATA (SATA) was introduced in August 2001, and fixed the issues with Parallel ATA.
SATAII, the more recent version, can run at 4Gb/s over an FC connection.

Data Access speeds

SATA uses two data channels - one for sending and one for receiving. The signals run phase reversed, so interference is self-canceling. This means it’s not necessary to twist the wires to avoid cross-signal inductance. However, there have been reports of data corruption on SATA disks, and its believed that this corruption is due to the unshielded nature of the SATA cables.

The maximum data transfer rate was increased to 150 MByte/s (1.2Gb/s). This is theoretical burst speed, and actual sustained speeds are likely to be nearer 75MB/s. Current production SATA disks (end 2003) are actually parallel ATA with a bridging chip, and so will not perform as well as native parallel ATA. This will improve when true SATA disks are produced.

Signaling Voltage

Serial ATA just requires a 500 millivolts signal. This fixes the issues with the 5-volt signaling requirement for the parallel ATA interface.

Signal error checking and correcting capabilities are improved, and end-to-end integrity of both transferred data and commands can be guaranteed across the serial bus.

SATAII Cabling

The Connection Cable now contains seven wires instead of 40. The Connectors measure just 8 mm wide, much smaller than the large and cumbersome 40-pin connectors required by parallel ATA. The thin, flexible serial cable can be up to 1 meter in length, which makes it easier to route inside the chassis. This, in turn means that housing designs can be smaller, or more ATA disks can be fitted inside a chassis.

Also, Serial ATA is backward compatible with the parallel ATA interface. Because the initial Serial ATA architecture changes the physical interface layer only, it maintains register and software compatibility with parallel ATA. No device driver changes are necessary and the Serial ATA architecture is transparent to the BIOS and the operating system.

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