Small Computer Systems Interface

SCSI is a bus for connecting peripherals to the computer, such as hard disk drives, CD drives and scanners that use this communication technology. At the time it was developed its main advantage was that peripheral control was at the peripheral itself and not at the interface. This idea was later used on IDE standard.

SCSI can have internal and external peripherals.

Since usually motherboards don’t have an on-board SCSI interface, it is necessary to install a SCSI board to have this bus on a PC. This board is normally called “SCSI host”.

The communication between the SCSI board and SCSI peripherals can be of 8- or 16 bits. For internal peripherals 8-bit communication uses a 50-wire flat-cable, while 16-bit communication uses a 68-wire flat-cable. Obviously 16-bit communication offers the double of performance compared to 8-bit communication if they are running under the same clock. 16-bit SCSI communication is also known as Wide SCSI.

It is possible to connect up to seven peripherals to an 8-bit SCSI board and up to 15 to a 16-bit SCSI board.

SCSI communications can use several transfer rates, starting at 5 MB/s and going up to 320 MB/s. In order to use the maximum performance provided by a SCSI peripheral you must use a board compatible to its transfer rate. For example, it is not good to have a 160 MB/s SCSI hard drive connected to an 80 MB/s SCSI host.

As you can see, the maximum transfer rate of SCSI standard is far higher than IDE, being the standard recommended for high-performance servers. Also, SCSI hard drives and interfaces targeted to servers allow a feature called hot swap, which allows you to exchange a defective hard drive with the server running. Another feature commonly associated to SCSI is RAID technology, even though in the last few years RAID for IDE hard disk drives became very popular.

The main problem of SCSI standard for end users is its price.

Another SCSI-related problem is the countless kinds of connectors available, accordingly to the application (internal, external, 8 bits or 16 bits).

Peripherals located at the ends of the SCSI cable must have their resistive terminator enabled, while the other devices must have their resistive terminator disabled. Enabling or disabling this terminator can be done thru a jumper, switch or software.

Figure 1: A SCSI host.

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