Upgrading and Repairing PCs

SCSI Versus ATA (IDE)

When you compare the performance and capabilities of ATA (AT attachment, sometimes also known as IDE for integrated drive electronics) interface and SCSI interface drives, you must consider several factors. These two types of drives are the most popular drives used in PC systems today, and a single manufacturer might make seemingly identical drives in both interfaces. Deciding which drive type is best for your system is a difficult decision that depends on many factors.

In most cases, you will find that an ATA drive using the same head-disk assembly as a SCSI drive from the same manufacturer performs about the same or outperforms the equivalent SCSI drive at a given task or benchmark. This is particularly true when using a single disk drive with a single-user operating system, such as Windows 9x/Me. More powerful operating systems, such as Windows NT, Windows 2000, or Windows XP, can more effectively use the command queuing and other features of SCSI, thus improving performance over ATA—especially when supporting multiple drives.

It is interesting to see that SCSI really evolved from ATA, or you could say that both evolved from the ST-506/412 and ESDI interfaces that were once used.

SCSI Hard Disk Evolution and Construction

SCSI is not a disk interface, but a bus that supports SCSI bus interface adapters connected to disk and other device controllers. The first SCSI drives for PCs were standard ST-506/412 or ESDI drives with a separate SCSI bus interface adapter (sometimes called a bridge controller) that converted the ST-506/412 or ESDI interfaces to SCSI. This interface originally was in the form of a secondary logic board, and the entire assembly often was mounted in an external case.

The next step was to build the SCSI bus interface "converter" board directly into the drive's own logic board. Today, we call these drives embedded SCSI drives because the SCSI interface is built in.

At that point, there was no need to conform to the absolute specifications of ST-506/412 or ESDI on the internal disk interface because the only other device the interface ever would have to talk to was built in as well. Thus, the disk-interface and controller-chipset manufacturers began to develop more customized chipsets that were based on the ST-506/412 or ESDI chipsets already available but offered more features and higher performance.

Today, if you look at a typical SCSI drive, you often can identify the chip or chipset that serves as the disk controller on the drive as being exactly the same kind that would be used on an ST-506/412 or ESDI controller, or as some evolutionary customized variation thereof.

Consider some examples. An ATA drive must fully emulate the system-level disk-controller interface introduced with the Western Digital WD1003 controller series IBM used in the AT. These drives must act as though they have a built-in ST-506/412 or ESDI controller; in fact, they actually do. Most of these built-in controllers have more capabilities than the original WD1003 series (usually in the form of additional commands), but they must at least respond to all the original commands that were used with the WD1003.

If you follow the hard-drive market, you usually will see that drive manufacturers offer most of their newer drives in both ATA and SCSI versions. In other words, if a manufacturer makes a particular 20GB ATA drive, you invariably see that the company also makes a SCSI model with the same capacity and specifications, which uses the same head disk assembly (HDA) and even looks the same as the ATA version. If you study these virtually identical drives, the only major difference you will find is the additional chip on the logic board of the SCSI version, called a SCSI Bus Adapter Chip (SBIC).

Figures 8.19 and 8.20 show the logic-block diagrams of an ATA and a SCSI drive from the same manufacturer. These drives use the same HDA; they differ only in their logic boards, and even the logic boards are the same except for the addition of an SBIC on the SCSI drive's logic board.

Notice that even the circuit designs of these two drives are almost identical. Both drives use an LSI (large scale integrated circuit) chip called the WD42C22 Disk Controller and Buffer Manager chip. In the ATA drive, this chip is connected through a DMA control chip directly to the AT bus. In the SCSI version, a WD33C93 SCSI bus interface controller chip is added to interface the disk-controller logic to the SCSI bus. In fact, the logic diagrams of these two drives differ only in the fact that the SCSI version has a complete subset of the ATA drive, with the SCSI bus interface controller logic added. This essentially is a very condensed version of the separate drive and bridge controller setups that were used in the early days of PC SCSI.

To top off this example, study the logic diagram in Figure 8.21 for the WD 1006V-MM1, which is an ST-506/412 controller used in early AT-compatible computers.

You can clearly see that the main LSI chip onboard is the same WD42C22 disk controller chip used in the ATA and SCSI drives. Here is what the technical reference literature says about that chip:

The WD42C22 integrates a high performance, low cost Winchester controller's architecture. The WD42C22 integrates the central elements of a Winchester controller subsystem such as the host interface, buffer manager, disk formatter/controller, encoder/decoder, CRC/ECC (Cyclic Redundancy Check/Error Correction Code) generator/checker, and drive interface into a single 84-pin PQFP (Plastic Quad Flat Pack) device.

The virtually identical design of ATA and SCSI drives is not unique to Western Digital. Most drive manufacturers design their ATA and SCSI drives the same way, often using these very same WD chips and disk controller and SCSI bus interface chips from other manufacturers. You now should be able to understand that most SCSI drives are "regular" ATA drives with SCSI bus logic added. This fact will come up again later in this chapter in the section "SCSI Versus ATA: Advantages and Limitations," which discusses performance and other issues differentiating these interfaces.

For another example, I have several old IBM 320MB and 400MB embedded SCSI-2 hard disks; each of these drives has onboard a WD-10C00 Programmable Disk Controller in the form of a 68-pin Plastic Leaded Chip Carrier (PLCC) chip. The technical literature states

This chip supports ST412, ESDI, SMD, and Optical interfaces. It has 27Mbps maximum transfer rate and an internal, fully programmable 48- or 32-bit ECC, 16-bit CRC-CCITT or external user defined ECC polynomial, fully programmable sector sizes, and 1.25 micron low power CMOS design.

In addition, these particular embedded SCSI drives include the 33C93 SCSI Bus Interface Controller chip, which also is used in the other SCSI drive that I mentioned. Again, there is a distinctly separate disk controller, and the SCSI interface is added on.

So again, most embedded SCSI drives have a built-in disk controller (usually based on previous ST-506/412 or ESDI designs) and additional logic to interface that controller to the SCSI bus (a built-in bridge controller, if you like). Now think about this from a performance standpoint. If virtually all SCSI drives really are ATA drives with a SCSI Bus Interface Controller chip added, what conclusions can you draw?

First, no drive can perform sustained data transfers faster than the data can actually be read from the disk platters. In other words, the HDA limits performance to whatever it is capable of achieving. Drives can transmit data in short bursts at very high speeds because they often have built-in cache or read-ahead buffers that store data. Many of the newer high-performance SCSI and ATA drives have 4MB or more of cache memory onboard. No matter how big or intelligent the cache is, however, sustained data transfer is still limited by the HDA.

Data from the HDA must pass through the disk controller circuits, which, as you have seen, are virtually identical between similar SCSI and ATA drives. In the ATA drive, this data then is presented directly to the system bus. In the SCSI drive, however, the data must pass through a SCSI bus interface adapter on the drive, travel through the SCSI bus, and then pass through another SCSI bus interface controller in the SCSI host adapter card in your system. The longer route a SCSI transfer must take makes this type of transfer slower than the much more direct ATA transfer.

The conventional wisdom has been that SCSI always is much faster than ATA; unfortunately, this wisdom is usually wrong. This incorrect conclusion was derived by looking at the raw SCSI and ISA bus performance capabilities. A 16-bit Ultra4 SCSI drive can transfer data at a claimed 320MBps, whereas an UltraATA/133 drive can transfer data at 133MBps. Based on these raw transfer rates, SCSI seems to be faster, but the raw transfer rate of the bus is not the limiting factor. As discussed previously, the actual HDA and disk-controller circuitry place limits on this performance. The key figure to check is what is reported as the internal or media transfer rate for the drive. For example, here are the specs on two similar drives (one ATA and one SCSI):

ATA Drive
Drive	Maxtor DiamondMax+ 60 ATA
Interface	UltraATA/100
Platters	3
Data heads	6
Capacity	60GB
Areal density (max)	15.4Gb/sq. in.
Rotational speed	7,200rpm
Internal cache buffer	2MB
Interface transfer rate	up to 100MBps
Media transfer rate	up to 57MBps
Sustained data transfer rate	28.5MBps–57MBps
Average sustained transfer rate	42.75MBps

SCSI Drive
Drive	IBM Ultrastar 73LZX SCSI
Interface	Ultra4 (Ultra320) SCSI
Platters	6
Data heads	12
Capacity	73.4GB
Areal density (max)	13.2Gb/sq. in.
Rotational speed	10,000rpm
Internal cache buffer	4MB
Interface transfer rate	up to 320MBps
Media transfer rate	up to 58MBps
Sustained data transfer rate	29.8MBps–58MBps
Average sustained transfer rate	44.4MBps

Note that although the SCSI drive can claim a much higher external transfer rate of 320MBps as compared to the ATA drive's 100MBps, in actuality these drives are almost identical performers. The real specs to look at are in the last two lines of the tables, which detail how fast data can actually be read from the drive. In particular, the sustained transfer rates are the true transfer rates for reading data from these drives; as you can see, they are very close indeed. The rotational speed, track density, and things such as internal buffers or caches mainly influence the true transfer rate of a drive.

Then you have to factor in that the SCSI drive listed earlier would cost 50%–100% more than the ATA counterpart. Not to mention the cost of springing for an Ultra4 host adapter to plug the drive into. Decent SCSI adapters, especially Ultra-SCSI versions, can easily cost $300 or more. When you consider that the UltraATA interface is built in to most modern motherboards for free, you can see that, depending on your needs, spending a ton of extra cash for SCSI can be a significant waste of money.

Now to present the counterpoint: Although these two drives are basically equivalent, the ATA version was one of the top ATA drives offered at the time, whereas the SCSI drive listed was only a middle-of-the-pack performer. IBM and others make SCSI drives that have rotational speeds of 15,000rpm, increased track density, and larger buffers. Of course, these drives cost even more. So, the bottom line is that if you must have the absolute best-performing drives, by all means get the top-of-the-line SCSI drives and Ultra SCSI Wide adapters. However, if you want something that is literally 1/3 to 1/4 the cost and 3/4 or more the performance, stick with ATA.

Performance

ATA drives currently are used in most PC configurations on the market because the cost of an ATA drive implementation is low and the performance capabilities are high. Serial ATA drives also compare favorably in cost and performance to SCSI. In comparing any given ATA and SCSI drive for performance, you have to look at the capabilities of the HDAs involved.

To minimize the variables in this type of comparison, it is easiest to compare ATA and SCSI drives from the same manufacturer that also use the same HDA. You will find that in most cases, a drive manufacturer makes a given drive available in both ATA and SCSI forms. For example, most hard drive companies make similar SCSI and ATA drives that use identical HDAs and that differ only in the logic board. The ATA version has a logic board with a built-in disk controller and a direct AT bus interface. The SCSI version has the same built-in disk controller and bus interface circuits and also an SBIC chip. The SBIC chip is a SCSI adapter that places the drive on the SCSI bus. In essence, virtually all SCSI drives actually are ATA drives with the SBIC chip added.

SCSI Versus ATA: Advantages and Limitations

Modern operating systems are multitasking, and SCSI devices (with all their additional controller circuitry) function independently of one another, unlike ATA. Therefore, data can be read and written to any of the SCSI devices simultaneously. This enables smoother multitasking and increased overall data throughput. The most advanced operating systems, such as Windows NT/2000/XP, even allow drive striping. A striped drive set is two or more drives that appear to the user as one drive. Data is split between the drives equally, again increasing overall throughput. Increased fault tolerance and performance are readily implemented and supported in SCSI drive arrays.

Ultra4 (Ultra320) SCSI drives offer advantages when compared with ATA. Ultra320 SCSI is 140% faster than UltraATA/133, which has a maximum data rate of 133MBps. Ultra320 SCSI also fully supports multitasking and can significantly improve system performance in workstations and servers running Windows NT, 2000, or XP. ATA limits cable lengths to 18 inches, effectively eliminating the ability to connect remote or external devices, whereas Ultra4 (Ultra320) SCSI allows external connections of up to 12 meters or more in length. Also note that ATA allows only 2 devices per cable, whereas Ultra320 SCSI can connect up to 15 devices. Finally, the domain validation feature of Ultra320 SCSI enables noise and other problems on the bus to be handled properly, whereas with ATA, if a problem occurs with the connection (and that is more common at the UltraATA/100 and 133 speeds), the ATA drives simply fail.

ATA drives have much less command overhead for a given sector transfer than do SCSI drives. In addition to the drive-to-controller command overhead that both ATA and SCSI must perform, a SCSI transfer involves negotiating for the SCSI bus; selecting the target drive; requesting data; terminating the transfer over the bus; and finally converting the logical data addresses to the required cylinder, head, and sector addresses. This arrangement gives ATA an advantage in sequential transfers handled by a single-tasking operating system. In a multitasking system that can take advantage of the extra intelligence of the SCSI bus, SCSI can have the performance advantage.

SCSI drives offer significant architectural advantages over ATA and other drives. Because each SCSI drive has its own embedded disk controller that can function independently from the system CPU, the computer can issue simultaneous commands to every drive in the system. Each drive can store these commands in a queue and then perform the commands simultaneously with other drives in the system. The data could be fully buffered on the drive and transferred at high speed over the shared SCSI bus when a time slot was available.

Although ATA drives also have their own controllers, they do not operate simultaneously, and command queuing is not supported. In effect, the dual controllers in a dual-drive ATA installation work one at a time so as not to step on each other.

ATA does not support overlapped, multitasked I/O, which enables a device to take on multiple commands and work on them independently and in an order different from which they were received, releasing the bus for other devices to use. The ATA bus instead waits for each command to be completed before the next one can be sent.

As you can see, SCSI has some advantages over ATA, especially where expansion is concerned, and also with regard to support for multitasking operating systems. Unfortunately, it also costs more to implement.

One final advantage of SCSI is in the portability of external devices. It is easy to take an external SCSI CD-ROM, tape drive, scanner, or even a hard disk and quickly move it to another system. This allows moving peripherals more freely between systems and can be a bonus if you have several systems with which you might want to share a number of peripherals. Installing a new external SCSI device on a system is easier because you normally will not need to open it up.

Of course, moving an external SCSI device from one system to another is not nearly as easy as it would be with a USB or FireWire (IEEE-1394a) device, which is one reason the latter two (especially USB) are catching on in the mainstream market. USB and IEEE-1394 (FireWire) devices are Plug and Play, allowing you to attach or remove the device with the power on. Plus, no device ID or other configuration is necessary. Narrow and Wide SCSI devices must be attached with the device and system power turned off; they also must be configured to use a unique device ID and have proper termination set for existing and new devices.

Recommended SCSI Host Adapters, Cables, and Terminators

For SCSI host adapters, I usually recommend Adaptec. Its adapters work well and come with the necessary formatting and operating software. Windows 9x/Me, Windows 2000/XP, and even OS/2 have built-in support for Adaptec SCSI adapters. This support is a consideration in many cases because it frees you from having to deal with additional drivers.

Standard or Fast SCSI is adequately supported by the now-obsolete ISA bus, but if you are going to install a Wide SCSI bus—or especially an Ultra, Ultra2, or Ultra160/320 bus—you should consider a PCI host adapter. This is because ISA supports a maximum transfer speed of only about 8MBps, whereas a Fast-Wide SCSI bus runs up to 20MBps. Even faster versions such as Ultra3 (Ultra160) or Ultra4 (Ultra320) SCSI run up to a blazing 320MBps! In virtually every case, a local bus SCSI adapter would be a PCI bus version, which is supported in all current PC systems. For maximum performance, you should use a 64-bit-wide, 66MHz version of the PCI slot if your host adapter supports this version of the PCI standard.

Like all modern PCI adapters, plug-and-play is supported, meaning virtually all functions on the card can be configured and set through software. No more digging through manuals or looking for interrupt, DMA, I/O port, and other jumper settings—everything is controlled by software and saved in a flash memory module on the card. Following are some features found on recent SCSI cards:

• Complete configuration utility built in to the adapter's ROM

• Software-configurable IRQ, ROM addresses, DMA, I/O port addresses, SCSI parity, SCSI ID, and other settings

• Software-selectable automatic termination (no resistors to pull out!)

• Enhanced BIOS support for up to 15 drives

• No drivers required for more than two hard disks

• Drive spin-up on a per-drive basis available

• Boots from any SCSI ID

Adaptec has full PnP support on all its SCSI adapters. These adapters either are automatically configured in any PC that supports the PnP specification or can be configured manually through supplied software in non-PnP systems. The PnP SCSI adapters are highly recommended because they can be configured without opening up the PC! All functions are set by software, and there are no jumpers or switches to attend to. Most peripheral manufacturers write drivers for Adaptec's cards first, so you will not have many compatibility or driver-support problems with any Adaptec card.

For SCSI cables, I recommend CS Electronics (http://www.scsi-cables.com), which can supply or custom manufacture virtually any SCSI cable or adapter. It can also supply a wide range of terminators, as can a company called East/West Manufacturing Enterprises, Inc. (formerly Aeronics), which is also worth a look. Visit its Web site at http://www.ewme.com.