Small Computer Systems Interface (SCSI) Explained

What is the history of SCSI, what is it's future?
SCSI (Small Computer Systems Interface) has been here for a long time. Since the early days SCSI has set a standard by which computers and devices could communicate. Transfer speeds at this time were typically 5MByte/sec. Over the years, to keep pace with computers, SCSI has evolved by bus speeds increasing and widening the data path. There are plans to increase the performance to 640MByte/sec, with increased manageability.

What are the different types of SCSI?

Single Ended SCSI is a kind of electrical signal where one wire carries the signal and another wire or shield is connected to electrical ground.

High Voltage Differential SCSI (HVD formally referred to as Differential SCSI) is a kind of electrical connection using two wires, one of which carries a positive signal (V) and the other a negative signal (-V). At the receiver the difference between signals is measured yielding a signal proportional to V. This is intended to cancel out any noise induced in the wires, on the assumption that the same level of noise will have been induced in both wires. Often, twisted-pair wiring is used to try to ensure that this is the case. This type of SCSI is generally used where long distances of up to 25m are required between end devices. This hardware is very expensive and now almost obsolete.

Low Voltage Differential SCSI (LVD) is the latest technology allowing even faster transfer speeds. This works in a similar fashion to HVD except with lower voltages and a single integrated circuit reducing the overall cost.

SCSI# Connector used
SCSI-I 50 Way Centronics or 25 Way D
SCSI-II 50 Way Half-Pitch D, 50-Way Half-Pitch Centronics or 50-Way Centronics
SCSI-III 68 Way Half-Pitch D (2 variations: with either clip or screw fastenings)
SCSI-IV never existed
SCSI-V 68 Way Very High Density Centronics

What are: Standard, Fast, Ultra, Ultra2, Ultra3, Narrow and Wide?

These specify different attributes of the SCSI standard. Some can be combined often doubling the transfer speed. The bus speed of each data bit is quoted in MHz, and the total data speed is quoted in Mega Bytes per second (MByte/sec).

Standard, Fast, Ultra, Ultra2 and Ultra3 and Ultra320 make references to the bus clock frequency which is related to the data speed. Standard being 5MHz, Fast being 10MHz, Ultra at 20MHz, Ultra2 at 40MHz and Ultra3/Ultra160 also at 40MHz and Ultra320 at 80MHz. Both Ultra3/Ultra160 and Ultra320 use a new feature called “Double Transition Clocking” which allows data to be transferred not only on the raising edge of a clock cycle, but also on the falling edge which doubles the transfer rate.

Narrow and Wide makes references to the bus width. When narrow, which can be omitted, is used the bus width is 8 bits. Wide SCSI has a bus width of 16 bits.

SCSI Bus speed and width

Terminology SCSI Bus Speed (MByte/sec) SCSI Bus Width (bits)
SCSI-I (2) 5 8
Fast SCSI (2) 10 8
Fast Wide SCSI 20 16
Ultra SCSI (2) 20 8
Wide Ultra SCSI 40 16
Ultra2 SCSI (2,4) 40 8
Wide Ultra2 SCSI (4) 80 16
Ultra3/Ultra160 SCSI (6) 160 16
Ultra320 320 16

SCSI Bus Length and devices

Terminology Max. Bus Lengths, meters (1) Max. Device
Single- ended LVD Differential HVD
SCSI-I (2) 6 (3) 25 8
Fast SCSI (2) 3 (3) 25 8
Fast Wide SCSI 3 (3) 25 16
Ultra SCSI (2) 1,5 (3) 25 8
Ultra SCSI 3 - - 4
Wide Ultra SCSI - (3) 25 16
Wide Ultra SCSI 1, 5 - - 8
Wide Ultra SCSI 3 - - 4
Ultra2 SCSI (2,4) (4) 12 25 8
Wide Ultra2 SCSI (4) (4) 12 25 16
Ultra3/Ultra160 SCSI (6) (4) 12 (5) 16
Ultra320 (4) 12 (5) 16

1. The listed maximum bus lengths may be exceeded in point-to-point and engineered 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, then 12 meters operation is possible at this speed. However, if any 4. 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 Single-ended is not defined for speeds beyond Ultra
5. HVD (Differential) is not defined for speeds beyond Ultra2
6. After Ultra2 all new speeds are wide only