SCSI Connectivity

SCSI (Small Computer System Interface) is a standard based on parallel technology that currently provides the fastest data input/output (I/O) interface for small computers. In addition to the super fast speed, SCSI also offers the ability to daisy chain multiple devices for easy access.

SCSI-I standards were established in 1985 that specified 5 Mbps speed on an 8-bit bus and allowed up to 7 devices to be daisy chained. The initial application for SCSI-I was for data intensive hard drives. When the 16-bit bus was introduced, the term Wide SCSI was used and when the speed was doubled to 10 Mbps, the term Fast SCSI was used. SCSI-II standards came out in 1993 that specified 10 Mbps speed. In response to a growing demand for matching faster microprocessors, SCSI-III soon followed and increased I/O speed to 20 Mbps. In 1996 the Ultra SCSI standard was established which increases I/O speed to 49 Mbps. The Ultra 2-SCSI standard can double I/O speed to 80 Mbps. The new Ultra 160 doubles this speed once again to 160 Mbps.

LVD (Low Voltage Differential) has the same characteristics as SCSI, but runs on 3.3V DC instead of 5V DC. LCD generally allows higher data rates than standard SCSI III and is less sensitive to electromagnetic noise, which allows safe lengths of up o 12M. LCD devices have transmission speed of 80 Mbps and LVD, SE devices, a speed of 160 Mbps.

Finally SCA (Single Connector Attachment) is used on some high end SCSI hard disk drives. This connector carries both the data and power in a single unit and has 80 pins. Generally, SCA to SCSI I, II or III adapters are required to attach such devices to standard SCSI controller cards.

SCSI Bus Lengths

STA - SCSI Trade Associations

Notes:

1. The listed maximum bus lengths may be exceeded in point-to-point and engineering applications.
2. Use of the word "narrow" preceding SCSI, Ultra SCSI or Ultra2 SCSI is optional.
3. LVD was not defined in the original SCSI standards for this speed. If all devices on the bus support LVD, the 12M operation is possiable at this speed. However, if any device on the bus is singled-ended only, then the entire bus switches to single-ended mode and the distances in the single-ended column apply.
4. Single-ended is not defined for speeds beyound Ultra.
5. HVD (Differential) is not defined for speeds beyound Ultra2.
6. After Ultra2 all new speeds are wide only.

Internal Cables:

Internal LVD drivers, whilst using the same connector as SCSI III and SCSI Ultra, require higher specification twisted pair ribbon cable to sustain the higher transmission speeds of this device. Therefore LVD cables are different to SCSI-III/Ultra cables. It is important to note that SCSI III, SCSI III Ultra 160, LVD and LVD SE all use the same connector.

Terminators

Two terminators are needed to terminate a SCSI chain, one at each end. SCSI controller cards have built-in terminators leaving the last peripheral on the chain to be terminated. For non-differential SCSI, which covers the majority of cases, the following are recommended.

• Use passive SCSI terminators if the total chain distance is less than six feet and the I/O speed 10 Mbps or less.
• Use active SCSI terminators if SCSI-I and SCSI-II devices are used and the total length of the chain is 20 feet or less.
• Use forced perfect terminator (FPT) if the total length is more than 20 feet, more than 5 devices are chained or when passive or active terminators cannot do the job.
• Use differential terminator only when the SCSI card and devices are designed for differential transmission.