SCSI

SCSI (Small Computer Systems Interface) is a parallel data bus standard that connects storage devices, printers, and other peripherals to a host computer or controller. Unlike IDE, which was designed primarily for internal drives, SCSI was built as a general-purpose interface capable of chaining multiple devices on a single cable, each with its own address from 0 to 15 (or 0 to 31 on Wide SCSI variants).

The standard specifies both the electrical signals and the command protocol. Early SCSI ran at 5 MHz with 8-bit data (later 16-bit on Wide variants) and delivered 5 MB/s throughput. Ultra2 SCSI doubled this to 40 MB/s, while Ultra3 (also called SCSI-320) reached 320 MB/s. These speeds made SCSI the preferred choice for servers and high-performance workstations through the 1990s and early 2000s.

A SCSI system requires three components: the host adapter (a card or integrated controller), the cable (either 50-pin or 68-pin depending on variant), and termination resistors at both ends of the chain to prevent signal reflection. Improper termination remains one of the most common field failures, causing data corruption or system lockups. Hot-swapping was theoretically possible on SCA (Single Connector Attachment) drives in server environments, though not all implementations handled it reliably.

SCSI's complexity and higher cost compared to IDE eventually worked against it. As SATA (Serial ATA) matured in the 2000s, it offered comparable or superior speed, simpler cabling, native hot-swap support, and vastly lower component costs. By 2010, SCSI had largely disappeared from general computing, though it persisted in legacy systems and some specialized industrial automation controllers.

For anyone maintaining older equipment, SCSI knowledge remains relevant. Distinguishing between narrow and wide variants, understanding termination requirements, and knowing which command sets a drive supports can mean the difference between a working system and an expensive dead-stock machine.