SCSI - Computer Busses - page 154

7.3 SCSI interface

In its standard form the small computer systems interface (SCSI) standard uses a 50-pin
header connector and a ribbon cable to connect to up to eight devices. It overcomes the prob-
lems of the IDE, where devices have to be assigned as a master and a slave. SCSI and fast
SCSI transfer one byte at a time with a parity check on each byte. SCSI-II, wide SCSI and
Ultra SCSI use a 16-bit data transfer and a 68-pin connector. Table 7.3 lists the pin connec-
tions for SCSI-I (single-ended cable) and Fast SCSI (differential cable) and Table 7.4 lists
the pin connections for SCSI-II, wide SCSI and ultra SCSI.

Table 7.3 SCSI-I and Fast SCSI connections

Single-ended cable Differential cable

Pin Signal Pin Sig nal Pin Signal Pin Signal

1 GND 2

D0

1 G ND 2 G ND

3 GND 4

D1

3 + D0 4 - D0

5 GND 6

D2

5 + D1 6 - D1

7 GND 8

D3

6 + D2 8 - D2

9 GND 10

D4

8 + D3 10 - D3

11 GND 12

D5

11 + D4 12 - D4

13 GND 14

D6

13 + D5 14 - D5

15 GND 16

D7

15 + D6 16 - D6

17 GND 18

D(PARITY)

17 + D7 18 - D7

19 GND 20 GND 19 D(PARITY) 20 - D(PARITY)

21 GND 22 GND 21 DIFFSEN 22 GND

23 RESERVED 24 RESERVED 23 RESERVED 24 RESERVED

25 Open 26 TERMPWR 25 TERMPWR 26 TEMPWR

27 RESERVED 28 RESERVED 27 R ESER VED 28 R ESER VED

29 GND 30 GND 29 + ATN 30 - ATN

31 GND 32

ATN

31 G ND 32 G ND

33 GND 34 GND 33 + RST 34 - RST

35 GND 36

BSY

35 + ACK 36 - ACK

37 GND 38

ACK

37 + RST 38 - RST

39 GND 40

RST

39 + MSG 40 - MSG

41 GND 42

MSG

41 + S EL 42 - S EL

43 GND 44

S EL

43 + C/D 44 - C/D

45 GND 46

C/D

45 + R EQ 46 - R EQ

47 GND 48

R EQ

47 + I/O 48 - I/O

49 GND 50

I/O

49 GND 50 GND

7.3.1 Signals

A SCSI bus is made up of a SCSI host adapter connected to a number of SCSI units via a
SCSI bus. As all units connect to a common bus, only two units can transfer data at a time,
either from one SCSI unit to another or from one SCSI unit to the SCSI host. The great ad-
vantage of this transfer is that is does not involve the processor.