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SCSI FAQ.

What does the term "SCSI" mean?
The term "SCSI" is an acronym for Small Computer System Interface. In the 1970s the name was appropriate. Today, SCSI is used for PCs, workstations, servers, mainframes, supercomputers.

What is SCSI?
The Small Computer System Interface is a high-speed, intelligent peripheral I/O bus with a device independent protocol. It allows different peripheral devices and hosts to be interconnected on the same bus. Depending on the type of SCSI, you may have up to 8 or 16 devices connected to the SCSI bus. The number of devices can be dramatically expanded by the use of LUNs (Logic Unit Numbers). There must be at least one initiator (usually a host) and one target (a peripheral device) on a bus. There is a large variety of peripheral devices available for SCSI, including hard disk drives, floppy drives, CDs, optical storage devices, tape drives, printers and scanners to name a few. There are many implementations of SCSI starting with SCSI-1 to SCSI-2 to SCSI-3 including, Narrow, Wide, Fast, Ultra, Ultra-2 and Ultra160 SCSI. The SCSI specifications are approved and issued by ANSI and are developed by the X3T10 SCSI Committee.

What can I do with SCSI?
SCSI provides a high-speed, intelligent interface that allows an easy connection for up to 16 devices (8 devices for Narrow SCSI) on a single bus. These devices may be hard disks, floppy disks, CDs, tape drives, printers and scanners to name a few. Peripherals may be mounted in the computer or in an external enclosure. Total SCSI cable length is dependent on the type of SCSI.

What are the differences between SCSI-1 and SCSI-2?
The initial implementation of SCSI (now called SCSI-1) was designed primarily for Narrow (8-bit), single-ended, synchronous or asynchronous disk drives and was very limited relative to today's SCSI. It includes synchronous and asynchronous data transfers at speeds up to 5 Mbytes/sec. Only passive termination was defined. It did not include definitions of a device independent interface. The standard connectors are the familiar 50-pin, female, low-density (0.1 inch spacing), non-shielded connector (now termed the non-shielded Alternative 2, A-connector) for internal wiring and the equally familiar 50-pin, male, shielded "centronics" type connector for external wiring (now termed the shielded, Alternative 2, A-connector). This "centronics" type connector is frequently called the "SCSI-1 connector". 5 Mbyte/sec SCSI is termed "Slow" SCSI. SCSI cable lengths may be up to 6 meters (20 ft) for Slow SCSI. Even before X3.131-1986 was officially accepted by ANSI, the SCSI committee went to work on improving it.

Released by the ANSI Committee as specification IEEE X3.131-1994, SCSI-2 is also a complete, stand-alone document. Arguably the most significant addition of SCSI-2 is the expanded definition of the common command set (CCS) providing a common software interface for all disk drives and many peripherals other than disk drives. SCSI-2 defines the differential interface and the 16-bit and 32-bit "Wide" data bus; doubles data throughput to 10 Megatransfers per second (called "Fast" SCSI), which translates to 10 Mbytes/sec for Narrow (8-bit) SCSI and 20 Mbytes/sec for Wide (16-bit) SCSI; adds the smaller 50-pin, high density, micro-D connector (termed Alternative 1, A-connector); and terms all 50-pin cables "A" cables. This 50-pin high-density connector is commonly called the "SCSI-2 connector". SCSI-2 recommends active terminators in place of passive terminators for the single-ended bus. Backward compatible to SCSI-1. Note that in SCSI-2 the 16-bit bus requires two cables (one "A" cable and one "B" cable) to make a connection. This seriously limited growth of the Wide bus. SCSI-2 maximum recommended single-ended SCSI cable length is up to 3 m (10 ft) for Fast SCSI. Differential cable length is 25 m (82 ft) for Fast or Slow SCSI.

What are the differences between SCSI-2 and SCSI-3?
SCSI-3 changes the complete SCSI document structure and is no longer one document but a collection of documents, each with its own revision number. Some of these documents are the SCSI Primary Command (SPC) set layer, SCSI Block Commands (SBC) for hard disk interface, SCSI Stream Commands (SSC) for tape drives, SCSI Controller Commands (SCC) for RAID arrays, Multimedia Commands (MMC) , Media Changer Commands (MCC) and the SCSI Enclosure Services (SES) commands.

What is the difference between single-ended and differential SCSI?
Single-ended and differential are two methods of placing SCSI signals on the cabling. Single-ended uses one wire driven against ground and the signal is the voltage difference between that wire and ground. The differential interface drives two wires. The signal is the voltage difference between the two wires. Single-ended and differential are not directly compatible. (It should be noted that HVD and LVD are also not directly compatible). They can be interconnected by the use of a SCSI expander called a Single-ended to Differential Converter. Single-ended cable lengths are 6 to 1.5 meters (20 to 5 ft), decreasing with increasing data throughput, while differential (HVD and LVD) offers cable lengths to 25 meters (82 ft), regardless of the speed of the bus.

What is meant by "Narrow" SCSI?
Narrow SCSI is the term that is used for 8-bit SCSI. It can usually be identified by 50-pin connectors.

What is meant by "Wide" SCSI?
Wide SCSI is the term that is used for 16-bit SCSI. It can usually be identified by 68-pin connectors. From SCSI-2 until the SPI-3 document in SCSI-3, this term also applied to 32-bit SCSI. SPI-3 obsoleted the 32-bit SCSI bus.

What is Wide Ultra SCSI?
Ultra SCSI, defined in the SPI-2 document of SCSI-3 offers a maximum data throughput of 20 Mbytes/sec for Narrow (8-bit) SCSI. Ultra Wide SCSI is the 16-bit version that offers 40 Mbytes/sec data transfers. Ultra Wide single-ended SCSI has a maximum cable length of 1.5 m (5 ft) with more than 4 active IDs and 3 m (10 ft) with 4 or fewer active IDs. Ultra Wide differential SCSI has a maximum cable length of 25 m (82 ft).

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