Make way for the Serial ATA interface

The new disk drive interface is designed for PCs as well as low-end servers and network-attached storage file servers.

By Jerry D. Kachlic

Information-generation, sharing, and in turn, storage, show no signs of slowing down. In fact, ongoing advances and adoption of new technology for the data-communications industry have fueled greater demand for faster data-transfer speeds and increased performance in both business and personal applications, from servers to notebook computers.

In late August, the Serial ATA Working Group announced a new disk drive standard. Serial ATA, developed by a working group led by Intel, APT Technologies, Dell, IBM, Maxtor, and Seagate, is the next-generation storage interface intended to meet increased bandwidth and performance demands for PCs today and leave enough room for future data rate increases. This new technology is also expected to find a place in low-end servers.

ATA, or Advanced Technology Architecture, has been the main storage bus since the early days of personal computers. As PC system performance increased, the ATA bus stayed a step ahead by continuing to double in speed. Continued improvements in speed and the relatively low cost of ATA devices resulted in dominance in the PC market. These same attributes have attracted attention in other storage markets where SCSI is the main standard. SCSI has always been viewed as a higher-performance bus, but recent advancements in ATA have significantly narrowed and, in some cases, erased the performance gap.

The Serial ATA connection system provides a direct connection between the device and backplane.
Click here to enlarge image

To maintain its performance gains, a serial version of ATA was developed. Serial ATA was created as a backward-compatible system to replace the parallel ATA standard. This was achieved by serializing/de-serializing communications between the host and target device. The ATA commands for the host and device, which are controlled by the ATAPI specification, are unchanged.

While the main goal of the Serial ATA Working Group was to replace parallel ATA in PC desktops, it was apparent that with some minor changes in the connector system, a viable replacement for some SCSI applications could be created. As Serial ATA 1.0-compliant devices become available, it is anticipated that they will start to challenge SCSI in network-attached storage (NAS) and entry-level server designs.

The new standard offers many new features that make it attractive to the low-end server environment, including

Speed-Serial ATA uses two pairs of differential signal lines, one each for transmit and receive circuits. The specification maps out a 10-year growth plan with three generations of speed enhancements. The first generation will transmit at 1.5Gbps, followed by 3Gbps and 6Gbps.

Connectivity-Serial ATA is a point-to-point system, which means that there is a separate connection path between the host and each device. Parallel ATA was limited to four devices per host, with one device designated as the master. The master device controlled the other slave devices on the bus. Serial ATA eliminates this master/slave relationship, allowing direct communications with any device on the bus.

Ease of use-The new connection system designed for Serial ATA provides for a direct connection between the device and a backplane (see diagram). By adding a few key features into the connector design, adding and removing a device from a server are as simple as changing a tape cartridge. With the connector in a standardized position at the back of the device, an alignment feature allows for "blind mating" any standard device into its mating receptacle. To prevent ESD damage and protect the bus from power glitches, the connector incorporates a fixed series of contact positions. This pattern of sequentially spaced contacts ensures that when a device is inserted into a hot backplane, a pair of grounding contacts handles any ESD.

These features are not new to the server market. Serial ATA has leveraged existing technology as much as possible. The connector specifications closely match those of the SCA 2 connector system. In fact, a Serial ATA backplane design can be retrofitted into a SCSI server design with no additional hardware changes.

ATA has already found its way into the entry-level server market, and ATA drives are priced significantly lower than their SCSI counterparts. While parallel ATA performance is not on par with SCSI, it is sufficient for many applications. Serial ATA not only improves on performance but is intended to achieve cost parity with parallel ATA once it reaches the same volume level.

The Serial ATA standard also creates an industry-standard connector location for both 2.5-inch and 3.5-inch form-factor devices. It will no longer be necessary to use a custom adapter to modify the connections to handle vendor differences in the interface connectors. With commonality of 2.5-inch devices, there is potential for systems with much lower power consumption. An example would be a small NAS server with its own internal battery backup.

With the continued exponential growth of storage, Serial ATA provides a flexible platform to build lower-cost storage devices for new applications.

Jerry D. Kachlic is regional development engineer at Molex (www.molex.com) in Lisle, IL. He was the lead designer on the Serial ATA specification for the connector set and can be contacted at jkachlic@molex.com.

This article was originally published on February 01, 2002