SCSI

In the 1960s, IBM was working on its mainframe computer, the 360, and developed an innovative I/O bus for its time: it could communicate simultaneously with multiple peripherals. This bus evolved to become the OEM Channel, but when IBM submitted it to ANSI for standardization, the institute refused. The reason? Its overly proprietary nature. ANSI preferred to see the emergence of a parallel I/O bus that would meet commercial needs without depending on a single manufacturer.

The story took an unexpected turn in the early 1980s. At Shugart Associates, a hard disk drive manufacturer, a few engineers who would later found Adaptec developed a parallel interface they named SASI (Shugart Associates System Interface). This specification circulated among manufacturers and met with great success. In 1982, building on this adoption, it was presented to ANSI as the basis for a new standard. The institute seized the opportunity, formalized and extended the SASI specifications, but changed its name to SCSI to avoid any reference to a particular manufacturer. On June 18, 1986, SCSI officially became an ANSI standard.

SCSI stood out for its versatility: it could control a multitude of peripherals, from hard drives to printers to CD-ROM drives and scanners. Its architecture interconnected small computers with their intelligent peripherals, particularly storage systems. Performance was remarkable: up to 4 MB/s depending on implementation, with a range that reached 25 meters thanks to differential drivers and receivers.

What made SCSI clever was its logical rather than physical addressing protocol for all data blocks. Each logical unit could be queried to determine its capacity in blocks. This abstraction considerably simplified the management and replacement of peripherals. The protocol provided for the connection of multiple initiators and targets, with a distributed arbitration system built directly into its architecture.

The 1990s revealed an interesting divide in the computer world. Macintosh computers massively adopted SCSI to connect their peripherals, while PCs remained faithful to the IDE/ATA interface. This difference was not trivial: almost all Mac components connected via SCSI, whereas IDE only handled hard drives. On PCs, users had to juggle specific controllers for CD-ROM drives (often integrated into sound cards) and connect tape drives to the floppy disk controller. SCSI avoided this proliferation of interfaces through its single bus.

The race for performance constantly pushed the technology to new heights. The original SCSI operated with an 8-bit bus clocked at 5 MHz, which yielded approximately 5 MB/s. To go faster, two paths were available: widen the bus or speed up the clock. These successive improvements gave birth to SCSI-2, which introduced variants such as Wide SCSI (16-bit bus) and Fast SCSI (10 MHz clock).

HVD (High Voltage Differential) appeared in the SCSI-2 standard to push distance limits. This technique used an additional pin that guaranteed precise transmission, allowing cable runs of up to 25 meters. Ultra SCSI, which fell under the SCSI-3 standard, refined the concepts of SCSI-2. Throughput continued its ascent with Wide Ultra SCSI, Ultra2 SCSI, and Wide Ultra2 SCSI. Ultra3 SCSI simply doubled the clock frequency while maintaining its compatibility with SCSI-3.

SCSI's speed owed much to its underlying protocol, SCSI Block Commands (SBC). This system authorized continuous data transmission between peripherals with a single command. Take the example of an audio CD: the SCSI controller sent an SBC signal to the drive, which then streamed data to the host controller until the end of the disc or until receiving a stop signal. The drive could redirect its data directly to a hard disk, bypassing the controller and system bus to conserve bandwidth.

Yet, despite its undeniable qualities, SCSI ultimately lost the consumer market battle to IDE/ATA. The arrival of ATAPI (AT Attachment Packet Interface) changed everything: IDE peripherals could now handle devices other than simple hard drives. This extension of the IDE standard, coupled with the high cost of SCSI controllers, precipitated the gradual abandonment of this interface in personal computers. But SCSI did not disappear: it retained its place in professional systems and servers, where its technical characteristics remained essential.

Innovation did not stop there. iSCSI enabled SCSI commands to be sent over local, wide area, or Internet networks, revolutionizing remote access and management of SCSI peripherals. The influence of this interface was also found in other technologies: IEEE-1394 (FireWire) relied on a subset of SCSI-3 specifications to control its peripherals, and FireWire hubs bore a striking resemblance to simplified SCSI adapters.

Today, SCSI's legacy still permeates modern computer storage. Its logical addressing approach and command architecture have inspired the interfaces that succeeded it. Its ability to efficiently manage peripherals and its robustness in professional environments have made it a foundational technology.