What is this SCCSI anyway?? SCSI, pronounced "scuzzy", is an acronym for Small Computer Systems Interface. It refers to a parallel hardware interface that allows for the connection of up to 15 peripheral devices to a single SCSI host adapter.

The SCSI standard was developed by Apple for their Macintosh computers but can be found in many other types of hardware, particularly servers and other enterprise level products, and is commonly used for attaching devices such as disk drives, backup systems, scanners and even specialised printers.

SCSI interfaces provide for faster data transmission rates than standard serial and parallel ports at up to 80 megabytes per second. Because you can attach many devices to a single SCSI port, so that SCSI is really an I/O (In/Out) bus rather than just an interface.

SCSI is an ANSI standard, but there are many variations of it so two SCSI interfaces may be incompatible with each other. These incompatibilities can be physical, because SCSI supports several types of connectors, or due to sytem and llength issues. Although not all devices support all levels of SCSI, the evolving SCSI standards are generally backwards-compatible. That is, if you attach an older device to a newer computer with support for a later standard, the older device will work at the older and slower data rate.

SCSI I, II, III, LVD and V

SCSI-1 / SCSI I

SCSI-1, the original SCSI Standard, was approved in 1986. It supports transfer rates of up to 5 Mbps and up to 7 devices on an 8-bit bus (not including the controller card). The most common type of connector for SCSI-1 is the Centronics 50, also called Telco 50 or Amphenol 50 (for external use). Internally, SCSI-1 is always run on Dual-Row Socket (F) connectors on a 50 conductor ribbon cable.

SCSI-2 / SCSI II

Approved in 1994, SCSI-2 introduced optional 16 and 32 bit buses called "Wide SCSI". The transfer rate, normally 10 Mbps, can be pushed up to 40 Mbps when combined with Fast and Wide SCSI. SCSI-2 usually uses a Micro-D 50 pin connector with side clips, also known as the Mini-50, Micro-50, and Micro DB50 for external cables. Internally it is run on the same 50-pin ribbon cables as is SCSI-1.

SCSI-3 / SCSI III

SCSI-3 commonly uses a 68-pin ribbon cable for in-cabinet connections, and a 68-pin shielded twisted-pair for external connections. Unlike SCSI-1 and SCSI-2, the internal and external 68-pin connectors can be interconnected. The most common bus width for SCSI-3 is 16-bit with transfer rates of 20 Mbps.

SCSI LVD

LVD SCSI can offer transfer rates up to 160 MB/sec with LVD SCSI, under optimal conditions (good quality SCSI card, LVD-compliant cabling, and proper termination). The cabling must be the proper type of twisted-pair cabling to support high-speed LVD signals.

SCSI-5 / SCSI V

SCSI 5 uses the VHDCI connector externally and can implement faster speeds than earlier specifications.

SCSI Implementations

At the time of writing he following varieties of SCSI are currently implemented:

Technology Name	Maximum Cable Length (meters)	Clock Speed	Maximum Speed (MBps)	Maximum Number of Devices
SCSI-1	6	5Mhz	5	8
SCSI-2	6	10Mhz	5-10	8 or 16
Fast SCSI-2	3	10Mhz	10-20	8
Wide SCSI-2	3	10Mhz	20	16
Fast Wide SCSI-2	3	10Mhz	20	16
Ultra SCSI-3, 8-bit	1.5	20Mhz	20	8
Ultra SCSI-3, 16-bit	1.5	20Mhz	40	16
Ultra-2 SCSI	12	40Mhz	40	8
Wide Ultra-2 SCSI	12	40Mhz	80	16
Ultra-3 (Ultra160/m) SCSI	12	40Mhz	160	16
Ultra-320 SCSI	12	80Mhz DDR	320	16
Ultra-640 SCSI	12	160Mhz DDR	640	16

SCSI Cabling

Leads Direct has a huge range of SCSI cables always in stock, and can offer technical advice prior to purchase. To browse the range please visit our e-Shop and review the SCSI Cables product category.

Internal SCSI cables are usually ribbon cables that have multiple 68 pin or 50 pin connectors. Common configurations range from two connectors (device and host) up to 8 connectors (host plus 6 devices and terminator) or even more...

External SCIS cables are round, shielded, and only have two connectors i.e. one on each end. There are many different connectors, and many combinations of connector used to connect disparate devices.

SCSI Connectors

For more detailed information about SCSI Connectors please see our SCSI Connectors page