The Drive Way - ATA vs. SCSI

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The computer hard drive exists because most of your data needs to be stored in something more permanent than RAM. Even if you turn off the computer for days or years, the data on the hard drive will sit there waiting to be accessed. The following are the two most common types of hard drives being used today, ATA and SCSI:

• Advanced Technology Attachment (ATA)

  The official name given by the American National Standards Institute (ANSI) for the Integrated Device Electronics (IDE) or Enhanced Integrated Device Electronics (EIDE) hard drive, the most popular for desktop computers.

• Small Computer System Interface (SCSI)

  Apple Computer, the innovator who gave us the universal serial bus (USB), first introduced the faster and more expensive SCSI (pronounced “skuzzy”) drive. SCSI is a standard high-speed interface that enables peripherals to communicate faster than they could with previous interfaces.

Both the ATA and SCSI interfaces support a wide variety of devices, including hard drives, CD-ROM drives, tape drives, and RAID. The latest SCSI standard, Wide Ultra-3 SCSI, can transfer data at a rate of 160 megabits per second (Mbps); while the latest ATA standard, Ultra ATA133, bursts speeds of 133 Mbps. SCSI accommodates up to 16 daisy-chained devices on a single 12-meter cable connected to one channel on the motherboard, whereas ATA is limited to only two devices on a single cable and channel. The hard drive speeds go up to 15,000 RPM for SCSI and 7200 RPM for ATA. The most significant differences between ATA and SCSI are cost, complexity, and compatibility, as discussed next.

Cost Comparison

The built-in higher-performance features make SCSI drives more than twice as expensive as their ATA counterparts. Therefore, determining whether you really need that higher performance should be your first step in determining whether or not to choose or migrate to SCSI devices. You may discover that the less expensive ATA fills your requirements initially, if not forever, and you can always make a later assessment.

Complexity

Configuring SCSI drives is more complex than configuring ATA drives. An ATA device (hard drive, tape drive, or CD-ROM/DVD-ROM) must be either a masteror slave.ATA devices also provide the option of CableSelect,which configures the drive in the middle of the cable as the slave and the drive at the end of the cable as the master. However, the drives, cable, and IDE controller must all support the Cable Select jumper setting, or it will not work. I recommend avoiding this potential problem by simply configuring fixed jumper settings for master and slave. The boot hard disk can only be the master; whereas with SCSI, you can configure any device to be a boot device.

All SCSI devices need to be assigned a position in a chain, or an ID, usually from 0 to 7 or 0 to 16, depending on the type of SCSI drives. Most SCSI drives contain two sets of jumper blocks, much like the jumpers on a motherboard. A shunt is placed over a set of two pins to configure the SCSI drive’s ID. SCSI drives also need to be terminated, or the chain of devices must be closed. This usually
requires another jumper block and setting on the device.

On a SCSI chain, you must close (terminate) the ends of the chain (see Figure 6-1) and determine the order of the IDs between the ends of the SCSI chain (seeTable 6-1).

Device	ID	Configuration
Host adapter (#1 in Figure 6-1)	ID = 7	Terminator ON
SCSI hard drive 1 (#3)	ID = 2	Terminator OFF
SCSI hard drive 2 (#4)	ID = 1	Terminator OFF
SCSI hard drive 3 (#5)	ID = 0	Terminator ON

Table 6-1: Example of SCSI ID and Termination Configuration

The SCSI host adapter typically will provide you with a BIOS option to configure which device you can boot from. 

This chapter is from Build Your Own Server, by Tony Caputo (McGraw-Hill/Osborne, 2004, ISBN: 0072227281). Check it out at your favorite bookstore today. Buy this book now.

Next: Setting SCSI Jumpers >>
 

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