SCSI
1
SCSI Hard Drives
2
SCSI Hard Drives
SCSI drives are faster.
 More expensive.
 Require a separate host adapter.
 Connect in a daisy-chain.
 Allows multiple drives from a single host
adapter using a single PCI bus slot and a
single IRQ.
 Allows multiple commands.

3
ATA Drives
Motherboard
Slave
Master
4
ATA Drives
Secondary
Slave
Secondary
Master
Motherboard
Primary
Slave
Primary
Master
5
SCSI Drives
Motherboard
SCSI Host
Adapter
6
Internal SCSI Interface Cable
To Host
Adapter
To SCSI Drives
7
Internal SCSI Interface Cable
8
Motherboard
SCSI Drives
SCSI Host
Adapter
Terminators
9
SCSI Drives
Motherboard
SCSI Host
Adapter
SCSI
Controller
10
Connecting External SCSI
Devices
SCSI Connectors
Terminator
IN
OUT
IN
OUT
SCSI Device
SCSI Device
From SCSI
Host Adapter
OUT
SCSI Device
IN
11
Adding a SCSI Device
Terminator
OUT
IN
OUT
IN
New
Device
OUT
SCSI Device
IN
SCSI Device
SCSI Device
From SCSI
Host Adapter
OUT
SCSI Device
IN
12
SCSI Host Adapter
Internal
External
13
Two Internal and Two External
SCSI Peripherals
Internal
SCSI
Peripheral
#1
Host
Adapter
External
SCSI
Peripheral
#1
Internal SCSI
Peripheral #2
External SCSI
Peripheral # 2
Terminator
Terminator
14
SCSI-1



Bus Width
Data Transfer Rate
Devices supported
8-bits (1 byte)
5 MB/s
8
15
SCSI-1
Terminator
7
Host
Adapter
6
Terminator
SCSI Cables
5
4
3
2
1
SCSI Peripheral Devices
0
16
SCSI-2
Fast SCSI
 Fast Wide SCSI

17
Fast SCSI



Bus Width
Data Transfer Rate
Devices supported
8-bits (1 byte)
10 MB/s
8
18
Fast Wide SCSI



Bus Width
Data Transfer Rate
Devices supported
16-bit (2 bytes)
20 MB/s
16
19
SCSI-3
SPI-1
 SPI-2
 SPI-3
 SPI-4
 SPI-5

Ultra and Wide Ultra
Ultra2 and Wide Ultra2
Ultra3 or Ultra 160
Ultra 320
Ultra 640
20
SPI-1
Ultra SCSI (Sometimes called Narrow Ultra
SCSI or Fast-20 SCSI)
 Wide Ultra SCSI

21
Ultra SCSI



Bus Width
Data Transfer Rate
Devices supported
8-bit
20 MB/s
8
22
Wide Ultra SCSI



Bus Width
Data Transfer Rate
Devices supported
16-bit
40 MB/s
16
23
SPI-2
Ultra2 SCSI (Sometimes called Fast-40
SCSI or Ultra-40 SCSI)
 Wide Ultra2 SCSI (Sometimes called
Ultra-80

24
Ultra2 SCSI



Bus Width
Data Transfer Rate
Devices supported
8-bit
40 MB/s
8
25
Wide Ultra2 SCSI



Bus Width
Data Transfer Rate
Devices supported
16-bit
80 MB/s
16
26
SPI-3
Ultra3 SCSI or Ultra-160 SCSI



Bus Width
Data Transfer Rate
Devices supported
16-bit
160 MB/s
16
27
SPI-4
Ultra-320 SCSI



Bus Width
Data Transfer Rate
Devices supported
16-bit
320 MB/s
16
28
SPI-5
Ultra-640 SCSI



Bus Width
Data Transfer Rate
Devices supported
16-bit
640 MB/s
16
29
Ultra640
SCSI
Ultra320
SCSI
Time
Ultra160
SCSI
Wide
Ultra2
SCSI Ultra2 SCSI
Ultra
Wide
SCSI Ultra SCSI
Fast
SCSI
Fast
Wide SCSI
10
20
SCSI
5
40
80
Data Transfer Rates
(Megabytes per second)
160
320
640
30
Ultra640
SCSI
SPI-5
Ultra320
SCSI
SPI-4
Time
Ultra160
SPI-3
SCSI
Wide
Ultra2
SPI-2
SCSI Ultra2 SCSI
Ultra
Wide
SPI-1
SCSI Ultra SCSI
Fast
SCSI
Fast
Wide SCSI
SCSI-3
SCSI-2
SCSI-1
5MB 10MB
20MB
40MB
80MB
Data Transfer Rates
160MB 320MB 640MB
31
 Narrow
SCSI is 8-bits wide and
accommodates 8 devices.
 Wide SCSI is 16-bits wide and
accommodates 16 devices.
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One of the devices is always the
SCSI host adaptor.
Terminator
Terminator
0
SCSI
Host
Adapter
1
2
3
4
5
6
7
33
Terminator
Terminator
0
1
2
3
SCSI
Host
Adapter
4
5
6
7
Disabled
Terminator
34
Passive SCSI Terminators
TERMPWR
220-Ohm
Resistor
SCSI Bus Line
330-Ohm
Resistor
35
Active SCSI Terminator
TERMPWR
Voltage
Regulator
2.8V
SCSI
Bus Line
110-Ohm
Resistor
36
Forced Perfect Termination
(FPT)
A variation of active termination.
 Includes clamping diodes.
 Eliminates signal undershoot and overshoot.
 Contributes to longer cable lengths.

37
SCSI Addressing Scheme or
SCSI IDs
8
devices numbered 0 through 7
 16 devices numbered 0 through 15
38
SCSI Priority
A kind of “pecking order” to determine
which device gets first shot at using the
SCSI bus.
 Determined by the SCSI ID.

39
The SCSI ID determines the
priority of the SCSI device.
7 6 5 4 3 2 1 0
Highest
Priority
Lowest
Priority
40
SCSI ID determines priority
Terminator
Terminator
7
Host
Adapter
6
5
4
3
2
SCSI Devices
1
0
41
Customary SCSI IDs
SCSI Host Adapter
 Primary Hard Drive

ID 7
ID 0
42
SCSI Priority
Narrow SCSI
7 6 5 4 3 2 1 0
Highest
Lowest
Wide SCSI
7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8
Highest
Lowest
43
SCSI Bus Arbitration
Device waits for bus to become free.
 Device signals its intention to use the bus,
indicating its address.
 If a second device attempts to use the bus at
the same time, the device with the higher
priority wins.

44
Setting the SCSI ID
Each SCSI device must have a unique ID.
 The SCSI ID is set when a new SCSI device
is installed.
 The SCSI Host Adapter should be given
SCSI ID 7.
 May be set by hardware or in software.

45
Setting the SCSI ID
7
0
6
2
5
Jumpers
Slide Switches
1
4
3
Rotary Switch
46
Setting the SCSI ID with
Jumpers
0
1
2
3
4
5
6
7
47
IDE
SCSI
Wide
SCSI
48
Logical Unit Number (LUN)
A sub-level addressing scheme.
 Each SCSI ID can have up to 8 LUN
addresses.
 Narrow SCSI allows 7 SCSI IDs times 8
LUN addresses for 56 attached devices.
 Wide SCSI allows 15 SCSI IDs times 8
LUN addresses for 120 attached devices.

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SCSI IDs and LUNs
SCSI
ID 0
SCSI
ID 1
SCSI
ID 2
LUN0
LUN0
LUN1
LUN1
LUN2
LUN3
LUN2
LUN3
50
Centronics 50-pin SCSI
Connector
2 Rows of 25 Pins
51
Centronics 50-pin SCSI
Connector
52
DB-25 SCSI Connector
1
1
53
50-pin High-density SCSI
Connector
Latch
Latch
54
50-pin High-density vs.
50-pin Centronics
55
68-pin, High-density SCSI
Connector
(HD68)
56
68-pin SCSI Connector used
on an Internal Ribbon Cable
57
68-pin, Very High Density
Micro Centronics Connector
(VHDCI68)
58
80-pin Single
Connector Attachment
(SCA80)
59
50-pin Centronics
50-pin High Density
68-pin High Density
80-pin SCA
60
SCSI cable adapter
50-Pin
Connector
68-Pin
Connector
61
Using the SCSI cable adapter
16-bit Data
7
6
5
16-bit
Devices
8-bit Data
4
3
68-pin to 50-pin
Adapter
2
1
8-bit
Devices
0
62
Host
Adapter
68/50
Adapter
68-pin
Cable
50/68
Adapter
50-pin Cable
8-bit Data
68-pin
Cable
16-bit
Data
63
Mixing Ultra-2 and Ultra-3
SCSI Drives
Host
Adapter
Ultra-3
Ultra-2
64
SCSI Signaling
Single-ended (SE)
 Low Voltage Differential (LVD)
 High Voltage Differential (HVD)
 Low Voltage Differential/ Single-ended
Multimode (LVD/SE)

65
Single-ended (SE) Signaling
Used by “standard” or normal SCSI
 Low cost
 Low performance
 Prone to noise
 Also called “unbalanced” signaling

66
Single-ended (SE) Signaling
+V
0V
67
Single-ended (SE) signaling is
prone to noise problems.
EMI
Signal
+V
0V
Noise
68
Single-ended (SE) signaling is
limited to a maximum bus
length of 6 meters.
6 Meters
(About 20 feet)
7
6
5
4
3
2
1
0
69
Single-ended SCSI vs.
Maximum Bus Length
SCSI
Technology
Transfer Rate
Max. Bus
Length (Meters)
Standard
5 Mbps
6
Wide
10 Mbps
6
Fast
10 Mbps
3
Fast/Wide
20 Mbps
3
Ultra
20 Mbps
3 or 1.5
Ultra/Wide
40 Mbps
3 or 1.5
70
Differential Signaling
+V
0V
0V
-V
71
Differential signaling is less prone
to noise problems.
EMI
+V
0V
Noise
Signal
0V
-V
72
High-voltage Differential (HVD)
Low-voltage Differential (LVD)
73
The Problems with High-Voltage
Differential (HVD)
High-voltage circuits cost more than lowvoltage circuits.
 Not compatible with single-ended and
LVD devices.
 Will damage single-ended devices or LVD
devices on the same bus.

74
Low-voltage Differential (LVD)
Its differential signaling technique allows
bus lengths up to 12 meters.
 Requires an LVD terminator that is different
from the standard SCSI terminator.
 It uses voltages that are compatible with
single-ended SCSI devices.

75
Low-voltage Differential/ Singleended (LVD/SE) Multimode
Can communicate with SE devices
 If one or more SE device is attached, the
bus length restrictions are those of the SE
device, not the 12 meters of LVD devices
 Requires the LVD terminator to work in
the LVD mode.

76
SCSI Symbol Icons
SCSI
SE
SCSI
LVD
SCSI
DIFF
(HVD)
SCSI
LVD/SE
77
The SCSI icons are important
because:
They tell you if the device will work with
your system by indicating the signal type.
 Putting a high-voltage differential (HVD)
device in a bus with a single ended (SE) or
low-voltage differential (LVD) device will
cause damage.

78