ATA to Ultra ATA/66
Advanced ATA Storage Interface

• Introduction
• Interface History
• Understanding the Need for a Faster Disc Interface
• Technology Overview
• Performance Increase
• Cost Stabilization
• Backward Compatibility
• System Requirements
• Data Integrity and Reliability
• Conclusions
• More Information

Introduction

The PC industry is constantly searching for advanced technology. This equates to more disc space, faster performance, more memory, better displays – virtually every component is under relentless pressure to improve. Continual improvement for the disc drive industry means lower costs, improved reliability, higher capacity, and better performance. As PC performance increases, the performance of the hard drive, which is the central input/output (I/O) device of the PC, becomes increasingly important. Improvement in disc drive performance is a complex area and is measured using several components: seek time, rotational latency, internal transfer rate, cache, and interface speed.

Interface History

The hard drive interface is the path through which data travels between the PC and the hard drive. The original ISA-dependent ATA (IDE) interface was limited to about 4 Mbytes/sec in the beginning, but reached as high as 8 Mbytes/sec. Interface protocols, such as programmed input/output (PIO) and direct memory access (DMA) modes, were designed to take advantage of the new local bus architectures that replaced ISA. ATA interface modes have progressed from PIO to DMA and now Ultra DMA, giving data transfer rates from 8.3, 11.1, and 13.3 Mbytes/sec up to 16.6, 33.3, and now 66.6 Mbytes/sec.

The trends in the above chart show that several components have improved with the evolution of the ATA interface. Speed and functionality have made major strides over the years. Performance remains the most commonly considered attribute with interface developments, and Ultra ATA/66 makes burst data transfer rates of up to 66.6 Mbytes/sec possible.

Understanding the Need for a Faster Disc Interface

Ultra ATA/66 provides a low-cost, high-reliability, backwards-compatible solution to data transfer bottlenecks that slow overall system performance. As the data storage density (areal density) of disc drives and rotational speeds have increased, bottlenecks also increased, thus requiring the ATA interface to improve performance to attain compatible data transfer speeds. Potentially, such improvements benefit PC end-users by providing faster PCs -- applications run faster, graphics run more smoothly, and multimedia flows uninterrupted on the screen. Actual performance benefits depend on the total system design and the applications being used. However, the drive performance trend is clear: performance demands on desktop and mobile disc drives will continue to push data transfer rates higher.

Technology Overview

The original ATA interface is based on transistor-transistor logic (TTL) bus interface technology, which is in turn based on the old industry standard architecture (ISA) bus protocol. This protocol uses a data transfer method called asynchronous. Both data and command signals are sent along a signal pulse called a strobe, but the data and command signals are not interconnected. Only one type of signal (data or command) can be sent at a time, meaning a data request must be completed before a command or other type of signal can be sent along the same strobe.

Starting with ATA-2 a more efficient method of data transfer called synchronous is used. In synchronous mode, the drive controls the strobe and synchronizes the data and command signals with the rising edge of each pulse. Synchronous data transfers interpret the rising edge of the strobe as a signal separator. Each pulse of the strobe can carry a data or command signal, allowing data and commands to be interspersed along the strobe. To get improved performance in this environment, it is logical to increase the strobe rate. A faster strobe means faster data transfer, but as the strobe rate increases, the system becomes increasingly sensitive to electro-magnetic interference (EMI, also known as signal interference or noise) which can cause data corruption and transfer errors.

ATA-4 includes Ultra ATA which, in an effort to avoid EMI, makes the most of existing strobe rates by using both the rising and falling edges of the strobe as signal separators. Thus twice as much data is transferred at the same strobe rate in the same time period. While ATA-2 and ATA-3 transfer data at burst rates up to 16.6 Mbytes per second, Ultra ATA provides burst transfer rates up to 33.3 Mbytes/sec. The ATA-4 specification adds Ultra DMA mode 2 (33.3 Mbytes/sec) to the previous PIO modes 0-4 and traditional DMA modes 0-2.

ATA-5 includes Ultra ATA/66 which doubles the Ultra ATA burst transfer rate by reducing setup times and increasing the strobe rate. The faster strobe rate increases EMI, which cannot be eliminated by the standard 40-pin cable used by ATA and Ultra ATA. To eliminate this increase in EMI, a new 40-pin, 80-conductor cable was developed. This cable adds 40 additional grounds lines between each of the original 40 ground and signal lines. The additional 40 lines help shield the signal from EMI. The ATA-5 specification adds Ultra DMA modes 3 (44.4 Mbytes/sec) and 4 (66.6 Mbytes/sec) to the previous PIO modes 0-4, DMA modes 0-2, and Ultra DMA mode 2.

Performance Increase

Just as Ultra ATA doubled previous transfer rates from 16.6 to 33.3 Mbytes/sec, Ultra ATA/66 potentially doubles the Ultra ATA burst transfer rate from 33.3 to 66.6 Mbytes/sec.

Cost Stabilization

Because Ultra ATA/66 is attained using improved firmware and electronic features, the end-user cost of an Ultra ATA/66 drive remains essentially the same as Ultra ATA drives. The user gets better performance for the same cost.

Backward Compatibility

Ultra ATA/66 is backward compatible with existing Ultra ATA and all legacy ATA systems. The Ultra ATA/66 drive can be connected into a legacy system as easily as before. Cables will continue to support the traditional 40-pin ATA signal without change.

System Requirements for Ultra ATA/66

To get the Ultra ATA/66 performance benefits, the following must be present:

• Compatible System – the system board must have a special Ultra ATA/66 detect circuit, and the system BIOS must support Ultra ATA/66.
• Compatible Operating System – the operating system must be able to handle Direct Memory Access (DMA). Win95 (OSR2) and Win98 comply.
• Compatible Cable – 40-pin, 80-conductor cable is required.
• Compatible Device – Seagate Ultra ATA/66 drives require an activation utility to toggle Ultra ATA/66 mode on and off.

Data Integrity and Reliability

Ultra DMA Modes 0-4 introduced an error-detection mechanism known as cyclical redundancy checking (CRC). CRC is an algorithm that calculates an order and value sensitive checksum used to detect errors in a stream of data. Both the host (controller) and the drive calculate a CRC value for each Ultra DMA burst. After the host-requested data is sent, the drive calculates a CRC value and this is compared to the original host CRC value. If a difference is reported, the host may be required to select a slower transfer mode and re-try the original request for data. (Note: CRC errors are detected and reported only when operating in an Ultra DMA transfer mode.)