Storage Interfaces

SCSI Operation

Where IDE/ATA is the most common interface for desktop/home computers, the Small Computer Systems Interface (SCSI) is the interface of choice for high-end workstations and servers. Originally created in 1979, SCSI was standardized in 1986.

SCSI offers some definite advantages over IDE/ATA, namely in the areas of performance and scalability. Because of the emphasis placed on performance, SCSI devices are generally more expensive than IDE/ATA devices. Additionally, SCSI devices require more configuration and planning prior to implementation.

The SCSI interface is a parallel interface. It is a shared-bus configuration that supports a wide variety of peripherals. The SCSI bus operates independently of the system CPU and the system bus through an integrated or plug-in controller. A SCSI controller can have multiple channels, each of which operates independently, and can have multiple devices daisy-chained together. Control of the SCSI bus is achieved through arbitration. During the arbitration process, devices on the bus assume the role of either initiator or target. Initiators are devices that send out commands; targets are devices that service those commands.

SCSI devices are connected to the SCSI controller by a SCSI cable. No more than two devices can communicate on the SCSI bus at any given time. The graphic below demonstrates how multiple SCSI devices are connected.

This is a sample of how multiple SCSI devices are connected.

Connection of Multiple SCSI Devices