Learn the difference between SCSI and IDE

Understanding the differences between SCSI and IDE is not easy, especially when many techs don't have any SCSI hands-on experience. Take a minute to learn about these standards so that you'll be ready when SCSI tries to take over your workstations.

Although the Small Computer Systems Interface (SCSI) standard has been around since the late 1980s, unless you've supported a network server, you probably don't have hands-on SCSI experience. While the Integrated Device Electronics (IDE) standard is still king of the workstation drive world, SCSI devices are growing in popularity and may soon be appearing on standard PCs.

This article looks at the basics of SCSI technology and outlines the key differences between the IDE and SCSI standards so that you’ll be prepared if SCSI devices become the next big thing in your corner of the IT world.

IDE: The essential workstation drive
The IDE interface is based on the IBM PC Industry Standard Architecture (ISA) 16-bit bus. The American National Standards Institute (ANSI) accepted IDE as a standard in 1990, under the name Advanced Technology Attachment (ATA). When the IDE standard was first introduced, many motherboards required separate controller cards to interface with IDE devices.

Almost all modern motherboards have two on-board IDE ports, with each port being able to control two IDE devices, for a total of four. If you want to add more than four IDE devices, you must install an additional controller card such as the one shown in Figure A. IDE ports have 39 or 40 pins and use the PCI bus to communicate with IDE peripherals.

Figure A
This ATA66 IDE controller card uses a PCI slot and has two IDE ports.

As of this article's printing, the Enhanced IDE (EIDE) is the standard for IDE drives. ANSI accepted the EIDE standard in 1994 under the name ATA-2 or Fast-ATA. This standard includes support for direct memory access (DMA) and multiple hard drives and CD-ROM drives through the AT Attachment Packet Interface (ATAPI).

Original IDE drives were unable to handle drive sizes larger than 528 MB due to their use of the cylinders, head, and sectors (CHS) method of addressing space. EIDE drives use logical block addressing (LBA) that allows a hard drive's sectors to appear to the OS as one long sequence of data with a concurrent address for each sector. Typically, when referring to an EIDE drive, the "E" is assumed and not mentioned.

The current IDE drive standard is the ATA 100 interface that supports data rates up to 100 MBps. In August 2001, Maxtor introduced its Ultra ATA 133 interface, branded Fast Drives. This interface is capable of transferring data at up to 133 MBps and is backwards-compatible with the current Ultra ATA 100 standard. Intel and other disk vendors, however, are backing the 150-MBps Serial ATA standard as the next-generation PC disk interface. Serial ATA is not compatible with current disk drives.

SCSI: Not just for drives
The SCSI-1 standard was originally designed by Apple and accepted by ANSI in 1986. As with the IDE standard, when SCSI was first introduced, most motherboards required SCSI controller cards to interface with SCSI devices. Unlike IDE, however, this remained the case until only recently. Some motherboards now come with built-in SCSI controllers, but these boards are generally used for network servers and are usually more expensive than workstation motherboards. Although some drives also come with built-in controllers, you still need a SCSI adapter card to interface with the motherboard. Figure B shows a SCSI controller card.

Figure B
This SCSI controller card uses a PCI slot and supports SCSI-1, SCSI-2, SCSI-3, and Ultra Wide SCSI devices. It has 50-pin and 68-pin internal connectors, along with one 68-pin external connector, and is capable of controlling 15 daisy-chained devices.

While hard drives and CD-ROM drives generally use IDE, a variety of devices, such as printers and scanners, use the SCSI standard. SCSI devices can bypass the CPU and send data directly to the system’s RAM by using direct memory access.

SCSI standards
SCSI-1's limitations soon prompted the development of the SCSI-2 standard, which allowed for greater functionality and increased the number of chainable devices to 15. As part of the SCSI-2 standard, Fast SCSI (a faster interface type) and Wide SCSI (which uses 16 bits instead of 8) were developed. The latest SCSI incarnation is the Ultra SCSI standard, which uses a higher clock speed and burst technology to transfer data more quickly than SCSI-1 or SCSI-2. Several variations of the Ultra SCSI standard are also available, including Ultra-2 SCSI, Wide Ultra-2 SCSI, and Ultra-3 SCSI. Ultra-3 SCSI devices are capable of sending data at 160 MBps over a maximum cable distance of 12 meters.

Cables, daisy chains, and termination
SCSI devices can be connected together in a parallel (or daisy-chain) method, using cables to connect one device to another. Daisy-chaining allows data to flow along the cables through each device and eventually into the computer. Using this method, the devices can even communicate with each other. The original SCSI standard allowed you to string up to seven devices together, while the newer standards allow daisy-chaining of up to 15 devices.

Because the signal passes through each device in the chain, the chain must be terminated at the end to prevent signal bounce (an echo that interferes with the original signal's clarity, causing interference and loss of signal). Current SCSI technology allows for signal termination in one of three ways: through a discrete terminating resistor placed on the output port of the last device, through a terminator built in to the last device, or by software. Figure C shows several types of discrete terminating resistors.

Figure C
SCSI terminators come in many configurations. From top to bottom: a 68-pin, external, female, single-ended, active terminator; a Centronics 50, male, passive terminator; and a 68-pin, internal, female, single-ended, active terminator.

Which should you choose?
Although SCSI-1 and SCSI-2 devices have come down in price, high-end SCSI devices are still significantly more expensive than their IDE counterparts. A 50-GB Ultra-2 SCSI hard drive costs around $500, compared to $100 for a 60-GB EIDE drive. The SCSI drive boasts a considerably higher data transfer rate, but is it worth the cost? For most workstations, standard EIDE drives are more than sufficient.

SCSI in the workstation?
When is the right time for a workstation to use SCSI devices? Which workstations in your organization use SCSI devices, if any? Do SCSI devices require more support time than IDE devices? Post a comment to this article and share your experiences.



Bill Detwiler is Managing Editor of TechRepublic and Tech Pro Research and the host of Cracking Open, CNET and TechRepublic's popular online show. Prior to joining TechRepublic in 2000, Bill was an IT manager, database administrator, and desktop supp...

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