SCSI is a standard interface bus, through which computers may communicate with devices such as fixed disks, CD-ROMs, printers, plotters, scanners, etc. The standard is fully described in ANSI standard X3.131-1986.
SCSI is a bus level interface, as compared to interfaces such as ESDI and ST-506, which are device level interfaces. With a bus level interface all the circuitry required to control the device is built into the device itself. This is the reason it is possible to attach different device types to a SCSI bus.
Up to eight SCSI devices can be attached to a single SCSI bus. The first of these devices is normally the SCSI attachment adapter, which would be installed in the PS/2 system unit and act as a bridge between the system unit bus (the Micro Channel) and the SCSI bus. A further seven SCSI devices can then be connected by means of the SCSI bus cable. Each of these physical devices can support up to eight logical devices. Each device attached to the SCSI bus has a device ID that is in the range 0 - 7. The attachment feature is usually given the ID of 7 making it the highest priority device on the bus. Figure "SCSI Subsystem Block Diagram" shows this diagrammatically.
SCSI supports features such as arbitration and disconnect/reconnect allowing several devices to operate concurrently and to share the bus. Data transfer across the SCSI can either be asynchronous or synchronous. Asynchronous data transfer requires that each byte sent across the bus be acknowledged before the next byte is transferred. When using synchronous mode, each byte of data must still be acknowledged but multiple bytes may be sent before any acknowledgements have been received.
The cable connecting the SCSI devices may be up to 20 feet (6 meters) in length, and carries 8-bit parallel data with a transfer rate of 5MB per second.