|Sunday, 17 December 2006|
The small computer system interface (SCSI) is a well known and widely used type of interface in the computer field. Communications are provided between a computer system and a device or provided between devices through the SCSI interfaces and by using a SCSI protocol. The SCSI interface uses logical rather than physical addressing for all data blocks. For direct-access devices, each logical unit may be interrogated to determine how many blocks it contains. A logical unit may coincide with all or part of a peripheral device. The SCSI system is a system level interface that provides an input/output (I/O) channel bus specification. The SCSI standard provides specifications for mechanical, electrical, and functional characteristics of the bus, including definitions of the physical characteristics of the bus conductors, the electrical characteristics of the signals that the conductors carry, and the meanings of those signals. The small computer system interface bus, along with its SCSI bus backplane, both of which are integral parts of a SCSI system, offer superb capability. The SCSI bus backplane may be a circuit card assembly included in low to high range server products. The SCSI backplane forms an integral part of a chassis of the host system. The backplane provides signals and pathways between the host system and up to six (or more) interconnected SCSI drives. The backplane functions to control the drives and to log system data. The SCSI backplane provides drive connectors into which drives or other peripheral devices may be plugged. The backplane typically features the ability to replace the drives while the host system power remains on. This is referred to as drive "hot swapping." Hot swapping involves automatically detecting drive presence, turning off or disconnecting power to the drive, removing it, replacing it with another drive, automatically detecting the other drive's presence, and sequencing power-on to the other drive by a delay ramp function. SCSI bus protocol is used in facilitating communication between one or more initiators and one or more targets. The interface protocol includes provision for the connection of multiple initiators (SCSI devices capable of initiating an operation) and multiple targets (SCSI devices capable of responding to a request to perform an operation). Distributed arbitration is built into the architecture of SCSI. Generally speaking, a computer is used as an initiator and a peripheral device, such as a disk drive, is used as a target. A single-ended interface is involved with the use of one signal in representing each control and data signal sent over the SCSI bus, while a differential interface is involved with the use of two signals (positive and negative polarity signals) in representing each control and data signal sent over the SCSI bus.
For over a decade the computer industry has relied on a SCSI bus cable to accommodate high-speed data transfer between a printed circuit board and an internal or external peripheral device. Each SCSI device is coupled to another SCSI device via a cable, which houses the SCSI bus. Hardware implementation of a communication bus which uses the SCSI protocol is generally done using a 50 conductor flat ribbon or round bundle cable of characteristic impedance of 100 ohm. The nominal distance is six meters in single ended mode and 25 meters in differential mode. Devices interconnected by a SCSI bus are daisy-chained together using a common 50-conductor cable. The cable comprises nine data conductors (eight for data and one for parity), nine control conductors, and other power and ground conductors. Optionally, a 68-conductor cable may be used to allow wider information transfers (data only) of 16 bits. The SCSI cables are daisy-chained from device to device and are permitted to reach a maximum of 6-meters in length. A small "stub cable" of up to 0.10 meters can be used to connect a peripheral device to the main cable. Generally, SCSI devices and busses that support an 8-bit data bus are referred to as narrow devices and SCSI devices and busses that support a 16- or 32-bit data bus are referred to as wide devices. To avoid signal degradation, the cables used for the SCSI bus are subject to stringent specifications. With a narrow, fast SCSI-2 system, the cable length cannot exceed three meters, and each drop (or stub) from the main cable to each device cannot exceed 0.1 meters. The distance between the stubs must be at least 0.3 meters. Each end of the SCSI bus must be terminated to attenuate signal reflections. A terminator absorbs the signals and minimizes reflections, typically by a technique known as impedance matching. Terminators are generally fixed or soft. Fixed, or permanent, terminators are always enabled to perform a termination function. Soft terminators can be enabled or disabled to perform a termination function as needed. Several SCSI cable types are known including ribbon cables and twisted-wire pair cables, each having an impedance between 90 and 140 ohms. The corresponding connector types include a 50-pin flat cable connector called an IDC header for ribbon cables and a 50-pin Centronics type connector for the twisted-wire pair cable. Typically each conductor is resistively coupled to a voltage of an inactive state. In the single-ended version, a ground signal conductor is provided for each data and control signal so that devices communicate over signal/ground pairs of wires. In the differential version, each signal includes a positive and a negative counterpart forming differential signal wire pairs. A SCSI expander is typically used to isolate, convert and expand the SCSI domain. An expander can isolate different SCSI domains allowing two or more devices to exist in electrically isolated domains.