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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Cables and Connectors ]

Wide (68-Conductor) Single-Ended Cables, Connectors and Signals

Wide cables are used for wide (16-bit) SCSI transfer modes. These are also sometimes called 68-conductor or 68-pin cables after the number of wires in the cable or pins in its connectors, respectively. Wide cables are formally called "P" cables in the SCSI standards, though there are several different types of "P" cables.

Note: Wide SCSI was defined in the SCSI-2 standard, many years after narrow SCSI was already established. The first wide SCSI implementations were designed to use a 68-pin "B" cable in combination with the regular narrow "A" cable. This combination was cumbersome and expensive, and very unpopular in the hardware industry. The "P" cable replaced the "A+B" combination and is now the standard for wide SCSI implementations.

Wide SCSI uses 68 signals, carried on the 68 conductors in the cable. These are organized into 34 pairs of two wires each. For single-ended SCSI, each pair generally consists of a signal and a signal return, which is the same as a ground line. The numbering of the signals is different for the cable conductors and the connector pins, though fortunately there is only one set of pin numbers, unlike narrow SCSI (see the narrow SCSI cable description for more). Here are the signals and numbering conventions for wide SCSI:

Signal

Pin #

Cable
Conductor
#

Pin #

Signal

SIGNAL RETURN

1

1

2

35

-DB(12)

SIGNAL RETURN

2

3

4

36

-DB(13)

SIGNAL RETURN

3

5

6

37

-DB(14)

SIGNAL RETURN

4

7

8

38

-DB(15)

SIGNAL RETURN

5

9

10

39

-DB(P1)

SIGNAL RETURN

6

11

12

40

-DB(0)

SIGNAL RETURN

7

13

14

41

-DB(1)

SIGNAL RETURN

8

15

16

42

-DB(2)

SIGNAL RETURN

9

17

18

43

-DB(3)

SIGNAL RETURN

10

19

20

44

-DB(4)

SIGNAL RETURN

11

21

22

45

-DB(5)

SIGNAL RETURN

12

23

24

46

-DB(6)

SIGNAL RETURN

13

25

26

47

-DB(7)

SIGNAL RETURN

14

27

28

48

-P_CRCA

GROUND

15

29

30

49

GROUND

GROUND

16

31

32

50

GROUND

TERMPWR

17

33

34

51

TERMPWR

TERMPWR

18

35

36

52

TERMPWR

(reserved)

19

37

38

53

(reserved)

GROUND

20

39

40

54

GROUND

SIGNAL RETURN

21

41

42

55

-ATN

GROUND

22

43

44

56

GROUND

SIGNAL RETURN

23

45

46

57

-BSY

SIGNAL RETURN

24

47

48

58

-ACK

SIGNAL RETURN

25

49

50

59

-RST

SIGNAL RETURN

26

51

52

60

-MSG

SIGNAL RETURN

27

53

54

61

-SEL

SIGNAL RETURN

28

55

56

62

-C/D

SIGNAL RETURN

29

57

58

63

-REQ

SIGNAL RETURN

30

59

60

64

-I/O

SIGNAL RETURN

31

61

62

65

-DB(8)

SIGNAL RETURN

32

63

64

66

-DB(9)

SIGNAL RETURN

33

65

66

67

-DB(10)

SIGNAL RETURN

34

67

68

68

-DB(11)

Note: "-P_CRCA" was "-DB(P0)" (parity zero) before the introduction of CRC.

The numbering of the conductors alternates between the left column of signal returns and the right column of signals, facilitating the creation of matched pairs within the cable, and also allowing for the creation of "partially twisted pair" LVD cables. The pins are numbered sequentially down the left column, and then the right column. This probably facilitates manufacturing in some way.

In practical terms, the following are the most common "P" cables used in the PC world for single-ended wide SCSI:

  • External High Density Cables: The older style of external wide cables uses the larger high density connector.
  • External Very High Density Cables: The newer style of external wide cables uses the very high density (VHDCI) connector. It is most common with the newest devices (though typically, VHDCI cables use LVD, not single-ended signaling.)
  • Internal High Density Cables: Only one general type of internal wide cable is used for single-ended signaling, a 68-wire ribbon cable with high density connectors.



Above, an external wide cable that has one high density
connector and one very high density connector. Below, an
internal wide cable with five high density connectors. Note the
integrated terminator on the internal connector (upper left).

Top image Computer Cable Makers, Inc.
Image used with permission.

As with narrow SCSI, the choice of external cable depends on the requirements of the hardware being used. There are cables available that have mixed connector types for special requirements, and also many kinds of adapters. Note that high-end SCSI now uses LVD, so LVD cables are required, which are similar in some ways to single-ended "P" cables, yet different. ;^) They are also, unfortunately, called "P" cables, so watch out for that! Note also that some wide SCSI implementations use single connector attachment (SCA) instead of discrete cables.

Next: Low Voltage Differential (LVD) Cables, Connectors and Signals


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