by Dinesh Thakur Category: Computer Terms

SCSI (pronounced "scuzzy," not "sexy") stands for small computer systems interface. SCSI is a standard for interfacing, or connecting, personal computers to peripheral devices (like scanners, hard disks, or CD-ROM players) and having them send information to each other.

The interface works through a SCSI cable that you connect to a SCSI port (which simply means that you connect a cord with a particular shape into the socket, or port, that matches). The computer itself usually has only one SCSI port, but you can attach up to seven other devices together in a chain of cables. Then your computer can send information to and receive information from these devices.

All Macintoshes (except the very earliest models, the 128 and 512) have a built-in SCSI interface. Only a very few PCs have SCSI interfaces.


SCSI-2 is a new standard for the small computer systems interface. It can transfer data faster, up to 10MBper second, whereas the first SCSI specifications were limited to about 4MBper second. SCSI-2is sometimes called "wide SCSI" because the interface can send 16 bits or 32 bits of information at a time. This means the speed could increase up to 20 and 40MBper second, as soon as the computers and printers all get the matching interfaces to take advantage of the new standard.

SCSI address

Computers that have a SCSI port (all Macs [except the ancient 128 and 512] and some pcs) can have up to seven devices attached to the computer, such as an external hard disk, a scanner, a CD-ROM player, etc. Since information travels through the cables to these separate devices, each one must have a different SCSI address so the information gets to the right place. A SCSI address is also called a SCSIID

A SCSI address is a number from 0 to 7. When you add a new device to your system, you determine the number yourself, either using software provided with the device or by flipping a little switch somewhere on the hardware (the manual will explain how; yes, you probably should read that manual). You can pick any number you choose for any peripheral device, just make sure the number for each address on the chain is different. The last device on the chain must be terminated. On a Mac, the computer itself always has a SCSI address of 7; the internal hard disk always has a SCSI address of 0 (zero). All other devices must have a number from 1 to 6. (My friend Clay is superstitious and always connects his SCSI devices in ascending numerical order. See termination for some very important information about connecting SCSIS.)

Because the computer always has a SCSI address of 7 and there cannot be more than one device with the same address, you cannot connect two computers to a common device at the same time, such as an external hard disk or a scanner or a CD-ROM player. You can't change the address  of the computer.

Don't confuse a SCSI address with any other kind of address, like a memory address or 32-bit addressing.

SCSI bus

The SCSI bus refers to the electronic connections between hardware that connect SCSI devices to each other- the connectors and the cables. Information (data) travels through the SCSI cables and the SCSI ports (which is where SCSI cables connect to the computer). They say this data travels on a bus. There are also other kinds of buses.

SCSI cable

A SCSI cable is the thick cable with the large, rectangular connectors on either end; this cable is used to connect SCSI devices together. For instance, if you want to connect a CD-ROM player or an external hard disk or a scanner to your computer, you need to hook it up with a SCSI cable

SCSI chain

A SCSI chain is when several SCSI devices are linked together with SCSI cables. Because there is usually only one SCSI port on the back of the computer, all the other devices you want to attach must be linked one to another in a chain. The first and last device on the chain must be terminated.

SCSI device

A SCSI device is an external piece of hardware that connects to the computer via a SCSI cable (those big fat cables with the big rectangular connectors on the ends). Scanners, CD-ROM players, and external hard disks (both regular and removable cartridge) are examples of SCSI devices. Mice, modems, keyboards, and most printers are not SCSI devices; they are considered serial devices and they connect to the computer through the serial or LocalTalk ports (those un intimidating little outlets, sometimes round and sometimes just like the outlets for our telephones).

SCSI port

A SCSI port is a plug-like socket on the back of a computer or on any SCSI peripheral device (such as a scanner or external hard disk) where you connect a SCSI cable. A SCSI port is a high-speed parallel port. It's standard on all but the oldest Macintoshes, on some IBMPS/Ss (model 65 and above), and on the IBMRs/6ooo. You can install a port on IBM PCs and compatibles as an expansion card (SCSI ports are different from standard PC parallel or printer ports). Even though there is only one port, you can attach up to seven devices through this one port, connected in a chain.

SCSI-to-SCSI cable

A SCSI-to-SCSI cable is a cable that connects directly from one SCSI device to another SCSI device (rather than from the computer to the device). For instance, you may want cables to connect from the scanner to the cartridge disk drive, then from the cartridge disk drive to the CD-ROM player. Through the SCSI port you can have up to seven SCSI devices communicating with your computer, each one hooked to the next with a SCSl-to-SCSI cable. You have to arrange the hardware like this because the computer has only one SCSI port, so it cannot connect directly to each device.

When you connect several SCSI devices to each other, you have a "SCSI chain," which is one form of a daisy chain. These devices are then said to be connected to the SCSI bus.

About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.C.A, MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.

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