What is SCSI cable used for?

SCSI (Small Computer System Interface) is an intelligent bus controlled by a microprocessor. Through SCSI cable, you can add up to 15 peripherals to your computers, such as scanners, printers, hard drives, and other peripheral devices. High-end single SCSI boards have two controllers and support up to 30 peripheral devices on a single expansion card. One advantage of SCSI is connecting multiple peripherals to a single host adapter, using only one slot in the bus.

Applications for SCSI cables

SCSI is now used more for large, industrial projects such as workstations, servers, and mainframes, and its use in desktop PCs has become rare. The advantage of SCSI cables in desktop PCs at the beginning was adding scanners, several other drives (e.g., CD-R, DVD-RAM, Zip drives), and hard drives to a SCSI cable chain. This has become less important with the popularity of alternative interfaces such as USB and FireWire.

SCSI is useful in network servers, where multiple hard drives can be easily set up in a RAID configuration. If one drive fails, it can be unmounted and inserted into a new drive without losing data while the system remains operational. This feature of RAID hardware is called hot swapping.

You can install a SCSI hard drive on a PC with one or more IDE disk drives. The IDE drive will still be the boot drive, while the SCSI drive will provide additional storage space. The IDE device will always be the default boot device, even if the SCSI hard drive is installed and configured with SCSI device number 0. For example, if both a SCSI hard drive and an IDE CD-ROM drive are present on a PC, the PC will always boot to the CD-ROM drive; the only way to fix this problem is to replace the IDE CD-ROM with a SCSI one.

Connecting SCSI devices

SCSI devices are usually daisy-chained together. External devices have two ports for linking input and output cables. Internal devices have one port for connecting to ribbon cables with multiple connectors. Some high-end SCSI cards may have multiple internal ports, allowing you to connect multiple ribbon cables.

Each SCSI device must have a unique ID number; typically, you can set these numbers by toggling a rotary switch on the external device or by setting a jumper on the internal device. The SCSI ID determines the order of the devices, from 7 to 0 and then from 15 to 8. The host adapter defaults to the highest priority, which is 7.

You must terminate the device at the end of the SCSI chain by setting a switch or plugging a resistor module into an open port. Usually, the host adapter is terminated by default. If the device is internally and externally connected, you must remove the host adapter termination and apply the term to the end of both chains.

Some adapters allow SCSI peripherals to be connected through the parallel port. The transfer rate of the parallel port is much lower than that of the SCSI host adapter, but it does provide a way to connect SCSI devices to a laptop computer. Not all SCSI devices will work on a parallel adapter, and some SCSI devices have parallel port adapters. Typically, the expected transfer rate is about 1MBps when using a SCSI-to-parallel port adapter.

SCSI Support

Windows 95, 98, NT, Me, 2000, XP, and most older Macintosh computers, provide internal support for SCSI, but Windows 3.1 and DOS do not. Newer Macintosh computers support FireWire instead of SCSI for a high-performance interface. You must add the appropriate SCSI driver to install SCSI on a Windows 3.1 or DOS computer.

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