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  Nothing Scuzzy about SCSI

With any computer, it is necessary to provide a method to hook hard drives and other devices to the motherboard. For most PCs, the common ways to do this are through bus and port connectors. Your hard drives and CDs are connected to the IDE connectors and restricted to 2 devices per connector. Printer, Scanner, Zip may be connected to the parallel port or the USB port. But what if you need more devices? Or a faster method of connecting these devices? Well, you can plug a SCSI card into your system and add up to 15 devices on the SCSI chain!

What is SCSI?

SCSI (pronounced scuzzy) stands for Small Computer System Interface and it is a bus interface used for hard drives, CD-ROMs, Scanners, and other peripherals. Since SCSI cards usually have both internal and external connectors, SCSI is more flexible in support of devices than the more common IDE interface. SCSI comes in multiple "flavors" because of the ongoing improvement to the bus and the speed it supports.

For a quick overview of SCSI, we have provided this table (courtesy of Boot Magazine):

Don't let the table intimidate you. It shows the overall history of SCSI devices, but doesn't have that much effect on buying them, because SCSI devices are backward compatible.

Terms	Bus Speed (MB/sec)	Bus Width (Bits)	Max Cable Length (Meters) SE 2      HVD 3   LVD 4			Devices supported
SCSI-1	5	8	6	25	-	8
SCSI-2	10	8	6	25	-	8
SCSI-3	20	8	1.5	25	-	16
SCSI-3	20	8	3	-	-	4
SCSI-3 1	20	16	6	25	-	16
SCSI-3 UW	40	16	-	25	-	16
SCSI-3 UW	40	16	1.5	-	-	8
SCSI-3 UW	40	16	3	-	-	4
SCSI-3 U2	40	8	-	12	12	8
SCSI-3 U2	80	16	-	25	25	2
SCSI-3 U2W	80	16	-	12	12	16
SCSI-3 U2W	80	16	-	25	25	2
SCSI-3 U3	160	16	-	12	12	16

1 Fast Wide SCSI 2 Single End Transceiver 3 High-Voltage Differential 4 Low-Voltage Differential

The number of devices includes the card, so for total number of peripherals, subtract 1 to get the number of devices you can add to your system.

There are a number of advantages to SCSI. First, every SCSI device has its own processor chip that allows it to do some actions without direct access to the CPU at the time. This means that SCSI devices are able to multi-task, meaning perform different tasks simultaneously, and that SCSI devices provide a lower CPU load. For servers, or high-end workstations, SCSI is the bus of choice for these very reasons, and because it can handle so many devices using 1 IRQ. For example, try to set-up a server with 15 hard drives using the IDE interface. It isn't easy!! With SCSI, it is a simple enough process. That multitasking ability and the speed of the bus also make the SCSI interface the bus of choice for high-end scanners and for CD burners. SCSI scanners are generally faster and can handle more data then other interfaces, and SCSI CD-ROMs and CD burners are able to handle direct duplication with less chance of buffer overruns.

In addition, the fastest hard drives in the world use the SCSI interface. Best known are the Seagate Cheetahs, but IBM also offers SCSI drives at 10,000 rpm.

Page 1: This page, introduction to SCSI
Page 2: Choosing SCSI devices
Page 3: Setting up a SCSI chain