A SCSI (Small Computer System Interface) drive interface is a standardized way for computers to communicate with peripheral devices such as hard disks, tape drives, CD-ROMs, and scanners. Instead of connecting each device directly to the CPU in a unique way, SCSI defines a common command set and electrical interface so multiple devices can share a single bus. Each device on a SCSI chain has its own ID, and the system can address them individually, allowing several drives (including disks and tape units) to operate efficiently on the same connection. SCSI supports relatively advanced features for its time, such as command queuing and the ability for devices to communicate with each other without constant CPU involvement.

SCSI originated in the late 1970s and early 1980s, evolving from earlier proprietary interfaces like Shugart Associates System Interface (SASI). It became formalized as a standard by ANSI in 1986 and quickly gained popularity in workstations, servers, and high-end PCs due to its flexibility and performance advantages over simpler interfaces like IDE. Over time, SCSI went through multiple generations—such as SCSI-1, SCSI-2, Ultra SCSI, and later Ultra320—each improving speed, cable length, and device capacity. Variants like Wide SCSI increased the data path width, while differential signaling improved reliability over longer distances, which was particularly useful in enterprise environments.

Although traditional parallel SCSI has largely been replaced in modern systems, its concepts live on in newer technologies. Interfaces such as Serial Attached SCSI (SAS) continue to use the SCSI command set while switching to faster, point-to-point serial connections. Even consumer technologies like USB mass storage and SATA drives borrow elements of SCSI’s command structure. Today, SCSI remains most relevant in enterprise storage—especially in servers and data centers—where reliability, scalability, and advanced control over storage devices are still critical.

A traditional (parallel) SCSI setup is a shared bus—one cable with multiple devices attached along it. Each device (hard disk, tape drive, etc.) is assigned a unique SCSI ID (typically 0–7 for narrow SCSI, 0–15 for wide SCSI). The host adapter (controller card) is also a device on the bus, often using ID 7 because it has the highest priority. Devices can be internal (on a ribbon cable inside the PC) or external (via a shielded cable), and both can exist on the same chain.

A critical detail is termination: the SCSI bus must be electrically terminated at both physical ends of the cable—no more, no less. Terminators prevent signal reflections that would otherwise corrupt data. Many devices have built-in terminators that can be enabled/disabled with jumpers or switches. Only the devices at the two ends of the chain should have termination turned on; everything in between must have it off. Cable length and quality also matter—faster SCSI versions required shorter, better-controlled cables.

An IDE interface is much simpler but less flexible than SCSI. Each IDE cable supports only two devices, configured as master and slave using jumpers. There’s no concept of a shared multi-device bus beyond that pair, and devices cannot independently arbitrate for control like SCSI devices can. IDE was common in consumer PCs because it was cheaper and easier to configure.

SATA (Serial ATA) replaced IDE and simplified things even further. Instead of sharing a cable, each drive gets its own point-to-point connection to the motherboard—no IDs, no termination, no master/slave settings. This makes setup very straightforward and improves reliability and speed. However, SATA lacks some of the advanced multi-device coordination and enterprise features that SCSI historically provided.

The NCR 5380 SCSI bus controller