SCSI Cabling Guide

SCSI was created to satisfy the need for a more flexible, faster, command-controlled interface for hard disk drives and other computer peripherals. Despite the term "small" in its name, SCSI is large. It is large in use, in market impact, influence, and unfortunately in documentation.

Early SCSI was simple, it had a few dozen commands. The standards effort that began with a 20-page specification in 1980 has grown to a 600-page extravaganza of technical information. Below is the more cable relevant information.

SCSI 1

The first work began on SCSI in the 1980's. Initially known as SCSI, it later became SCSI 1 to distinguish itself from later versions. At first the SCSI bus was designed for only two devices: the host bus (initiator) and the disk itself (the SCSI target) cables were very short and data was transmitted at 1.5 m/b per second or less. By today's standard this specification is low. but it was a significant improvement at that time. The connector used was a D50 with 3 rows of pins. Later versions the connector was a 50 way Centronics more modern SCSI 1 systems give a maximum of 8 devices including host 5 megabytes per second 8 bit wide parallel data bus. The standard was improved and to prevent delays, work begun on a SCSI 2 standard. Connectors used can be almost anything depending on the Manufacturer. No technical rationale can be offered as to where SCSI-1 ended and SCSI-2 began, or as to where SCSI-2 ended and SCSI-3 began. The justification is much more simple - you have to stop sometime and get a standard printed!. Popular interfaces never stop evolving, adapting, and expanding to meet more uses than originally envisaged. An ongoing standardisation effort began to extend the capabilities of SCSI-2.

SCSI 2.aka Fast SCSI2

A significant improvement from SCSI 1 data could now be transmitted at 10 Mb/per second; this is known as fast SCSI option two interfaces were designed, single ended and differential. The connector used was a Micro D 50 specifications for the cables, terminators etc. were included to improve the bus signal integrity. Many new features were added and a final spec was supposed to be submitted and approved by the end of 1992. Different manufactures use different features of the SCSI 2 standard. Connectors used include D50, micro D50 cents.

Fast-Wide aka SCSI 3

Very shortly designers saw the need for further improvements and wide or fast and wide SCSI evolved. Using 16-bits wide SCSI is capable of 20 Mb/per second with a 68 way micro d. New 32 bit systems were capable of speeds up to 40mb per second. These two systems can be identified as fast 20 & fast 40. New higher spec terminators are required to maximise the full potential of this system

Ultra 2 Wide SCSI (LVD)

Ultra2 SCSI (LVD), the latest implementation of the storage industry's dominant, proven interoperability technology, provides the capacity and performance demanded by today's data-intensive applications. It is easily integrated with current hardware and firmware. Compared with other alternatives available today, Ultra2 SCSI (LVD) prevents system performance from becoming I/O limited as drive throughputs increase. Ultra2 SCSI (LVD) integration performs faster drive buffer-to-host transfers, freeing the bus to accommodate more devices. The enhanced Ultra2 SCSI (LVD) is fully backward compatible, and uses the same physical environment as its predecessor. More important, it can be integrated without having to modify or change operating systems, thereby protecting the user's investment in operating system software and drivers. The literally millions of lines of SCSI software code already written can be used immediately with the next-generation Ultra2 SCSI (LVD) interface. Ultra2 SCSI (LVD) is well positioned for the future because it offers forward compatibility and significant room for growth to support faster microprocessors and drives. Other benefits include minimal SCSI software or protocol changes, with the support of 15 years of backward compatibility experience with SCSI. Currently, Ultra2 SCSI (LVD) is capable of achieving transfer rates of up to 80 MB/sec. Ultra2 SCSI (LVD) is low cost, easy to integrate, and an optimal solution for improved performance and increased device connectivity. It is available and fully implementable now.

Connectors on internal Ultra wide SCSI drives tend to be the Micro D68. (as used on fast wide) Some As well as this, some Ultra SCSI controllers provide the VHDCI interface (Very High Density Connection Interface) This is a Mini micro 68 way Centronics connection.

Currently existing SCSI standards are summarised in the table below.

SCSI TERMINATION

PASSIVE TERMINATORS (a.k.a. single ended) are mainly used on SCSI 1 single ended systems. The terminator has a 220/330 ohm resistor pack

ACTIVE TERMINATORS (a.k.a. regulated) are generally used on "loaded" SCSI systems (with many devices on the bus) a loaded bus system can cause a fast system to slow down from 10mhz to 5 MHz or less in order to transfer data reliably. This can be due to reflections caused by a mis-match between the cable and the terminator. Active terminators have a 110 ohm resister pack. If a single ended device with passive termination is experiencing problems, an active terminator can be a solution.

DIFFERENTIAL TERMINATORS Are used on differential SCSI systems only. This style of terminator consists of a 330/150/330 ohm resistor combination. This is a specific specification (?) And has not nor is it likely to be changed.

ACTIVE NEGATION TERMINATORS may replace the current active terminators. this will depend on the manufactures. Rather than produce 2 or 3 types of active terminator, they may consolidate their costs and introduce a "multi purpose" terminator.