understanding SCSI specifications

The Small Computer System Interface (SCSI) is the most popular interface for attaching high-performance disk drives, tape drives, optical drives, hard disk drives, scanners, printers, disk array subsystems (RAID) and CD-ROM drives.This vast array of products, coupled with the maturity and proven performance of parallel SCSI, make it very difficult for any new peripheral interface to garnish enough clout or gain enough market share to begin to compete successfully with parallel SCSI.

SCSI became an American National Standards Institute (ANSI) standard in 1986 and since then has evolved continuously. The data path has increased from 8 to 16 bits (Wide SCSI), and data transfer speeds have increased from 5 to 80 MBps (Ultra 2 SCSI). The next generation of SCSI further increases the transfer speed to 160 MBps [Ultra 3 SCSI/ Ultra 160/m].

Previous improvements in SCSI performance placed restrictions on the maximum SCSI cable length. Originally defined as a 6 m (20 ft) bus, SCSI reduced its length to 3 m (10 ft) and subsequently to 1.5 m (5 ft) with the advent of Fast SCSI and Ultra SCSI respectively.

Ultra2 SCSI, sometimes called Low-Voltage Differential SCSI (LVD), is the next generation in SCSI technology; it allows the maximum cable length to increase to 12 m (40 ft), which enables inter-rack connections, thereby overcoming the current limitations of certain intra-rack solutions. While Ultra2 SCSI allows for data throughput to 80 MBps, some implementations may not see a significant increase in system-level performance at these increased data rates. In spite of this, systems can now take advantage of the greater operating distances and improved system level configurability by deploying this new SCSI capability. Ultra2 is compatible with existing SCSI devices allowing older, non-Ultra2-capable controllers to be used with newer, Ultra2-capable devices and vice versa. This fosters a smooth and rapid transition to Ultra2 technology.

50-pin SCSI Connector 68-pin SCSI Connector 80-pin SCSI Connector

50-pin Connector
The Narrow or 50-pin interface was first implemented on SCSI-1 specfications allowing 8-bit data path between drive and CPU. Latter Fast SCSI devices was introduced with both 8-bit Narrow 50-pin and 16-bit Wide 68-pin connectors on the drives.

Macintosh platform and Mac Clones provided on board 50-pin SCSI support on all of their logic boards till Power Macintosh G3 (Beige) models. Some PCs and Workstation OEMs also provided on board SCSI support.

SCSI devices can also be connected via SCSI Card supporting different bus architectures, Currently PCI [ Peripheral Component Intrface] bus. PCI SCSI cards allow narrow or 50-pin drives or Wide 68-pin drives or SCA-80 pin drives with an SCA adapters to be connected to the computer with or with out on board SCSI support.

Also SCSI Cards allows the most current drives to be connected to the system data bus, thus establing data path for maximum I/O between the drive and the system provided right drive and SCSI Card is used. 68-pin Connector
The Wide or 68-pin interface was designed to provide double the data bandwidth of a Narrow or 50 pin drive.

Wide or 68-pin interface drives connect to a SCSI controller internally or externally, on board wide SCSI connection is rare, this feature enables the CPU to access data at 16-bit bandwidth. The Wide 68 pin and SCA 80 pin drives are both 16 bit wide and have the same data transfer rate, the only differnece is in drive connector attacment. Wide or 68-pin drive allows user to connect them in any desktop computer with a wide interface, allowing users to change drive operating parmeters and settings such as SCSI IDs, drive termination and other important drive settings.

Can Wide or 68-pin drives be used in Narrow or 8 bit applications?
Yes, with TransIntl 68 to 50 Adapters. 80-pin Connector
The SCA interface was designed to provide a standard connection for systems using hot swappable drives. SCA interface drives connect to a SCSI backplane that provides power, configuration settings such as SCSI ID, and termination of the SCSI bus.

How can an SCA interface drive be attached to a standard 50-pin or 68-pin SCSI host adapter?
An SCA adapter is necessary to attach an SCA interface drive to a standard SCSI host adapter. An SCA adapter should provide power and termination as well as jumper settings for SCSI ID and other drive features. Some drive models may offer configuration options on the
drive.

Please click for TransIntl SCA Adapter information.

Note:
SCA [Single connector assembly]
SCA-2 [enhancement that allows hot plugging]

Looking at how data rates have increased historically, it can be noted that SCSI performance has doubled approximately every five years. This is summarized below:

Note: STA [ SCSI Trade Association] SCSI Speed refers to the SCSI Device Data Xfer rate.

A key point in the evolution of parallel SCSI is that "backward compatibility", or "upgradability", has been maintained as each increase in parallel SCSI performance has occurred. This has allowed parallel SCSI technology to be enhanced without sacrificing existing or previous SCSI customers. Ease of upgrading has been a key enabler for the evolution from SCSI to Fast SCSI to Ultra SCSI. As a result, devices equipped with any of these three inter-faces can be mixed and coexist on a common SCSI bus, operating off that common bus at the speed specified by that device.

As the SCSI data rate has increased, the robustness of the systems equipped with SCSI has also increased because of advancements in things such as SCSI termination, controller I/O pad technology and physical configuration (i.e.cable length, impedance and loading). For example, the parameters for physical configuration of Fast SCSI is 16 loads and 6 meters of cable.

The Ultra SCSI standard, because it pays careful attention to loading and cabling, made further improvements. As such, single-ended Ultra SCSI supports a maximum cable length of 3 meters with 4 loads, or 1.5 meters with 8 loads. Ultra SCSI is also the last speed increase for single-ended active negation drivers.

The Ultra2 standard is the next step in this evolution and will overcome a number of issues associated with perfor-mance increases including connectivity and cable length.

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	Looking at how data rates have increased historically, it can be noted that SCSI performance has doubled approximately every five years. This is summarized below: Note: STA [ SCSI Trade Association] SCSI Speed refers to the SCSI Device Data Xfer rate. A key point in the evolution of parallel SCSI is that "backward compatibility", or "upgradability", has been maintained as each increase in parallel SCSI performance has occurred. This has allowed parallel SCSI technology to be enhanced without sacrificing existing or previous SCSI customers. Ease of upgrading has been a key enabler for the evolution from SCSI to Fast SCSI to Ultra SCSI. As a result, devices equipped with any of these three inter-faces can be mixed and coexist on a common SCSI bus, operating off that common bus at the speed specified by that device. As the SCSI data rate has increased, the robustness of the systems equipped with SCSI has also increased because of advancements in things such as SCSI termination, controller I/O pad technology and physical configuration (i.e.cable length, impedance and loading). For example, the parameters for physical configuration of Fast SCSI is 16 loads and 6 meters of cable. The Ultra SCSI standard, because it pays careful attention to loading and cabling, made further improvements. As such, single-ended Ultra SCSI supports a maximum cable length of 3 meters with 4 loads, or 1.5 meters with 8 loads. Ultra SCSI is also the last speed increase for single-ended active negation drivers. The Ultra2 standard is the next step in this evolution and will overcome a number of issues associated with perfor-mance increases including connectivity and cable length.