Sarasota PC Monitor
Tech Talk (02/03)
Serial ATA Hard Drives
by Brian K. Lewis, Ph.D.*
Member of the Sarasota Personal Computer Users Group, Inc.There is a significant change coming to hard drive technology- that's the serial ATA interface. In reality, it's already here and is being incorporated into new computers. The second specification for this interface was released in October 2002 and there are serial ATA drives on the market now. So what does this mean to the average computer user? Well, how about the words - Newer? Faster? Better? They do seem to be the primary slogan of the computer industry. Since you will be seeing these drives and their ads quite soon, let's take a look at this new technology.
First, let's clarify some of the terminology. You'll sometime see drives referred to as EIDE which stands for Enhanced Integrated Device Electronics. The IDE or EIDE term can be applied to any drive where the controller is located on the drive rather than on the motherboard. Both of these terms are used interchangeably with ATA. Technically speaking, the ATA is the interface standard and the IDE refers to the controller electronics on the drive. ATAPI is another term that is used in referring to CD-ROM, CD-RW, DVD and ZIP drives. It means ATA packet interface. Hard disks move data in the form of sectors or logical blocks and CD drives use data packets. In order to move CD drives from the more expensive SCSI interface, the ATA standard was modified to work with CD drives.
Ever since it was first released, the ATA specification has been undergoing constant change. One thing that has been consistent has been the drive size barrier of 137 GB. So the major improvements have been in bandwidth or data transmission speed. The original specification had a speed of 3.3 MB/s that has been increased to 133 MB/s in the ATA133 specification. However, this, along with the 137 GB size barrier seems to be the end for the parallel form of the ATA interface.
If you have ever looked inside your computer case and tried to identify the various components you would have seen two or more flat "ribbon" cables. These cables connect the hard drives, CD-ROM drives and/or floppy drives to the motherboard. All the information read/written to these drives has to travel through these cables. Depending on the age of your system the hard drive cables will have either 40 or 80 wires. The 80 wire cables are used with the newer ATA/100 and ATA/133 drives. The ATA refers to "Advanced Technology Attachment" and the numbers refer to the maximum data transfer speed. This speed is in megabytes per second (MB/s).
The transmission of data through these ribbon cables occurs in parallel mode. This means that transmission can occur over multiple wires at the same time. However, as transmission speed increases the high frequency signals develop problems such as cross-talk and signal reflection, among others. ATA drives are self-terminating. This is to reduce the problem with signal reflection. However, when a single drive is attached to the middle connector on the two-drive cable, reflection problems increase. When the change was made from 40 wire to 80 wire cables, even with the modified connector, problems increased. The smaller wire diameters increased the electrical resistance of the system and limited cable length to 18 inches. Folds in these ribbon cables also increase the probability of electrical problems and signal degradation. And, if the cable is crimped, transmission wires can be easily broken.
So why would parallel transmission be replaced by a serial method? After all, the most common serial ports on computers (the COM ports) are being replaced because they are too slow. However, their replacements are USB and FireWire, both of which are based on serial technology. Believe it or not, the fastest computer memory, RDRAM, is serial based, as is the networking protocol, Ethernet. Here we are talking about data transmission rates from 100-460 MB/s. In addition to the possible increases in speed, there is a great reduction in the number of wires required for the connection. USB 2.0 uses only four wires but has a maximum data capacity of 460 MB/s, So the design of the new Serial ATA (SATA) hard drives has an entirely new connector to the drive and the motherboard. The SATA connector has only 7 pins compared to the ribbon cable connection of 40 or 80 pins. In addition, the maximum cable length is 1-meter (39.37 inches) compared to the 18 inch maximum for parallel ATA. The cable is keyed to fit the motherboard and drive so there can be no error in the way it is connected. Data transmission requires only a 250-mV connection compared to the 5V used in parallel ATA. The reduction in the number of wires also allows an increase in signal wire diameter, which reduces resistance and impedance. This increases the speed of signal propagation.
One of the interesting changes in the design of the SATA system is the new power cable. Instead of the familiar 4 conductor power providing 5 and 12 Vdc, the new connector has 15 pins. This change allows an option of a third voltage, 3.3 V. Another key item is that there is a variation in the length of the connector pins. The ground pins are longer and will always connect first. This allows the devices to be electrostatically grounded before data or power connections are made. Thus, SATA drives can be safely hot-plugged just as is done with USB peripherals. Once a power connection is established, the drive will be initialized and establish a handshake with the host system. The drives are fully software compatible with current operating system software (Win9X, WinME, Win2K, WinXP, Linux, etc.) Official support for SATA hot-plugability probably won't appear in Windows before 2004. However, the backward compatibility of the SATA standard ensures the compatibility with current software.
So we have several advantages in converting to SATA drives: (1) complete compliance with the ATA protocol which means software compatibility; (2) improved data transmission with fewer errors resulting from fewer wires and reduced voltage; (3) improved air-flow within the case resulting from the smaller diameter cables; (4) hot-plug capability; and (5) an eventual increase to a 600 MB/s transmission speed. The current specification is for 150 MB/s with an increase to 300 MB/s expected during 2004. However, since current drives designed for the ATA133 specification don't come close to that in actual data transmission, initially, there won't be much of a real increase with SATA. The major problem is the speed of the PCI bus. A RAID array is the only current way of achieving speeds close to the maximum of the ATA specification.
So what are the shortcomings of this new technology? First, SATA allows only one device to be connected to a cable. That means if you currently have four ATA (or IDE) devices in your computer, you would need a separate cable and a separate motherboard connection for each of them. Currently, the motherboard needs only two connections as the ATA specification allows two devices for each connection. The SATA connection also eliminates the need for master/slave jumpers on the devices. Because the connections on the new drives are totally different from current drives, you can not connect to your current hardware. Manufacturers such as Adaptec and Promise, among others, have produced adapter cards that can be installed in a PCI slot. These can be connected to the new SATA drives and will co-exist with older ATA drives. Such adapters will be needed until new chipsets and motherboards are available which can take direct advantage of the SATA technology.
Second, there will be no real performance gains initially. The limitations of the PCI bus will restrict performance of the SATA drives to that of ATA133 drives. Until, the next generation PCI bus (PCI express?) arrives, there will be no real speed gains. However, the reliability of data transmission should increase with the conversion to SATA. This should reduce read/write problems and may show some gains by reducing data errors.
There is no real rush to replace your current hard drives with SATA drives. However, when you purchase your next computer, look into getting one with this newer technology. Overall, it looks like SATA will provide some definite benefits with the gains in transmission speed a promise for the future.
Dr. Lewis is a former University & Medical School professor. He has been working with personal computers for more than thirty years and can be reached via e-mail at: bwsail@yahoo.com or voice mail at 941/925-3047.
Copyright 2003. This article is from the February 2003 issue of the Sarasota PC Monitor, the official monthly publication of the Sarasota Personal Computer Users Group, Inc., P.O. Box 15889, Sarasota, FL 34277-1889. Permission to reprint is granted only to other non-profit computer user groups, provided proper credit is given to the author and our publication. We would appreciate receiving a copy of the publication the reprint appears in, please send to above address, Attn: Editor. For further information about our group, email: admin@spcug.org/ Web: http://www.spcug.org/The Sarasota Personal Computer Users Group, Inc. has 1,100+ members and was established in 1982. We are members of the Assoc. of PC User Groups (APCUG), the Florida Assoc. of PC Users Groups, Inc., and we are members of the America Online Ambassador Program.
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