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SCSI Guide - 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.


What about SCSI-I, SCSI-II,  SCSI-III, SCSI-IV and SCSI-V?

These generally refer to  the types of connectors used. With reference to external cables the  following can apply:

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 (2)

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/Ultra  160 SCSI (6)

(4)

12

(5)

16

Ultra320

(4)

12

(5)

16


Notes:
(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 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 beyond Ultra
(5) HVD (Differential) is not  defined for speeds beyond Ultra2
(6) After Ultra2 all new speeds  are wide only

Please click on the appropriate link to see  the full range of our SCSI Cables or Adapters.

 

 

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