HP Surestore 2200mx HP SureStore 220mx Optical Jukebox User's Guide - Page 82

Operating This Jukebox on a SCSI Bus A Brief Overview of SCSI A Brief Overview of SCSI General The Small Computer System Interface (SCSI), is a contention-based bus that accommodates different speed devices without impacting the devices with faster transfer speeds. This specification was defined by the American National Standards Institute (ANSI) in 1986. The specification defines both the physical medium and the command set used to transfer information. Later developments are known under the newer specifications, SCSI-2 and SCSI-3. The SCSI Bus The common SCSI buses are 50-pin and 68-pin. The 50-pin bus, which uses eight of its lines for data transmission, is called a "narrow" bus. The narrow bus can support eight devices. The 68-pin bus, which uses 16 lines for data transmission is called a "wide" bus. The wide bus can support 16 devices. Some devices may have 68-pin connectors, but operate with a "narrow" bus. (See the next section in this Appendix, "The SCSI Bus and This Jukebox". Initiators and Targets SCSI devices on the bus are either "initiators" or "targets." An initiator (usually the host computer) originates a transaction and the target (usually a peripheral device) fulfills the request. Initiators and targets identify themselves on the bus by a SCSI ID. The ID is designated by the user and is set electronically or manually depending on the device. In addition to identifying a device on the bus, the ID also determines the priority of the device during contention among the devices for use of the bus. The narrow SCSI bus, with its eight data lines, can communicate with eight devices that have IDs from 0 to 7. The wide SCSI bus, with its 16 data lines, can communicate with 16 devices that have addresses from 0 to 15. The host bus adapter, which links the host computer to the SCSI bus, is also a SCSI device (initiator), and is usually assigned an ID of 7. B- 2 Appendix B