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SCSI cables
Sunday, 17 December 2006

Digital computer systems are becoming more powerful, flexible, and distributed with each passing day. These advances are due to many factors, including improvements in software capabilities, in computer system architectures, and in semiconductor devices such as microprocessor and memory chips. One architecture that has often improved the flexibility and performance of small systems is the Small Computer System Interface (SCSI) standard. A small computer system interface (SCSI) is an interface unit which performs data input/output operations with a very low dependency on a central processing unit (CPU). The small computer systems interface (SCSI) is an American National Standards Institute (ANSI) standard communications bus that includes the electrical and logical protocol specification. The Small Computer System Interface (SCSI) defines an input/output bus and logical interfaces supporting the bus of interconnecting computer and peripheral devices. Computer systems typically comprise many discrete parts that communicate electronic data and control information among the various discrete parts. A bus is a collection of wires or traces over which the data and control information is communicated among the computer's discrete parts. The wires or cables are usually collected into "connectors," or female/male combinations of plugs. Whether a bus includes a collection of wires, a collection of traces, or both, depends on the nature of the discrete parts and their location in the computer system. The SCSI interface is a device independent input/output bus which allows a variety of peripheral devices to be connected to a personal computer system. A main objective of the standardized protocol adopted for SCSI parallel interface is to provide host computers with device independence within a class of devices. A computer is provided with SCSI capabilities via SCSI controller cards that connect to the host computer or via a SCSI chip that is integrated as part of the computer's motherboard. Devices that conform with the mechanical, electrical, timing, and protocol requirements (including the physical attributes of I/O buses used to interconnect computers and peripheral devices) of the SCSI parallel interface will inter-operate. This allows several different peripherals (hard disk drives, removable disk drives, tape drives, CD-ROM drives, printers, scanners, optical media drives, and so on) to be added at the same time to a host computer without requiring modifications to the generic system hardware. Once a computer is provided with SCSI capabilities, the user is able to connect either internal or external SCSI peripheral devices to the computer. Internally, SCSI ribbon cables are used to interconnect the peripheral devices to the computers controller card or a motherboard's connector. Externally, SCSI peripheral devices can be coupled to the computer's connector receptacle via an external SCSI cable.

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.