1. A+ Guide to Managing
and Maintaining Your PC
Fifth Edition
Chapter 22
All About SCSI

2. You Will Learn…

About basics of SCSI technology and
components

How SCSI hard drives compare to IDE drives

How to install a SCSI device

Troubleshooting tips for SCSI
2

3. SCSI Basics

Standard for communication between
subsystem of peripheral devices and system
bus



Bus can contain/be used by up to 7 or 15 devices
Bus requires controller embedded on motherboard
or host adapter inserted into an expansion slot
Faster, more expensive, more difficult to
install than similar IDE devices

Mostly used in corporate settings
3

4. The SCSI Subsystem

Enables two devices on SCSI bus to pass data
between them without going through CPU

Requires SCSI controller and unique SCSI IDs
assigned to each device, including host adapter

Components

Host adapters

Device drivers
4

5. SCSI Host Adapter
5

6. IDE versus SCSI Bus
Communication
6

7. Host Adapters

Manage all devices on SCSI bus

Support internal and external SCSI devices

Form a single daisy chain with devices
7

8. Considerations When Selecting
a Host Adapter

BIOS

Expansion slot

Bus mastering

SCAM-compliant
8

9. BIOS Considerations

Plug and Play compatibility

Built-in configuration utility

Software that configures termination
automatically and assigns system resources

Ability to configure SCSI devices using the
bus controlled by the adapter

Support for up to 15 peripherals
9

10. Expansion Slot Considerations

Host adapter must fit expansion slot
(eg, 16-bit ISA, PCI)
10

11. Bus Mastering

Bus master attached to PCI bus can access
memory and other devices without accessing
the CPU

SCSI host adapter does not require a DMA
channel
11

12. SCAM-Compliant

Can assign SCSI IDs dynamically at startup

Two levels

Level 1 requires that the devices – but not the host
adapter – be assigned an ID at startup by software

Level 2 requires that the host adapter – as well as
the devices – be assigned an ID at startup by
software
12

13. SCSI Device Drivers

Enable OS to communicate with a host adapter

Two popular types

ASPI (Advanced SCSI Programming Interface) –
more popular of the two

CAM (Common Access Method)
13

14. Variations in SCSI

Bus width

Signaling methods used on SCSI cables

Connectors used with SCSI cables

Termination

SCSI-1, SCSI-2, and SCSI-3
14

15. Bus Width

Narrow SCSI



8-bit data path
Uses a cable with a 50-pin connector (A cable)
Wide SCSI

16-bit data path

Uses a cable with a 68-pin connector (P cable)
15

16. Signaling Methods Used on
SCSI Cables
Single-ended (SE)
Differential
Send signal on pair of
twisted wires – one carries
voltage, one is a ground
 Less expensive than
differential
 Shorter maximum cable
length; data integrity is not
as great


Send signal on pair of
twisted wires – both carry
voltage; signal calculated to
be the difference between
the two
 Better signal accuracy and
greater reliability
 HVD and LVD
16

17. Signaling Methods Used on
SCSI Cables (continued)
17

18. SCSI Cables
18

19. Signaling Symbols
19

20. Connectors Used with SCSI
Cables

Not affected by signaling method used

Types vary in shape and pin density; can be
connected with adapters

Two main types:



50-pin (A cable)
68-pin (P cable)
Last connector on last device must be filled
with a terminator
20

21. SCSI Connectors
21

22. Multiple Connectors on the
SCSI Bus
22

23. Termination

Prevents echo effect from electrical noise and
reflected data at end of SCSI daisy chain

Required at each end of a SCSI chain
23

24. Ways to Terminate the End of a
SCSI Chain

Switch setting on host adapter that activates or
deactivates a terminating resistor on the card

Single SCSI connection or two connections

Resistor physically mounted on the device

Internal terminators that turn on or off with a
jumper setting on the device

Software-controlled termination
24

25. External SCSI Terminator
25

26. Terminators

Types

Passive terminators

Active terminators

Forced perfect terminators (FPTs)

Single-ended SCSI cables use passive
terminators, active terminators, and FPTs

Differential cables use either HVD or LVD
terminators
26

27. Major Versions of SCSI

SCSI-1 (Regular SCSI)




No common command set
8-bit data bus
7 possible devices
SCSI-2 (Fast SCSI)




Common command set
16-bit data bus
15 possible devices
Mandatory parity checking
27

28. Major Versions of SCSI
(continued)

SCSI-3 (Ultra SCSI)

Supports both parallel and serial data transmission

Supports FireWire connections

Data transfer of 320 MB/sec

Includes SPI standard
28

29. SCIS Standards
29

30. SCIS Standards (continued)
30

31. SCIS Standard Cable Specs
31

32. Comparing IDE and SCSI

IDE


May give better performance when using a single
disk drive with an OS like Windows 98 or
Windows Me
SCSI



Increases performance where heavy load is placed
on system and its components
Some OSs include increased support for SCSI
features and take better advantage of them
More expensive; better performance
32

33. Considerations When Choosing
Between IDE and SCSI Drives
IDE
SCSI
IDE supports only four internal Supports internal and external
devices
devices; allows additional ones
Separate IRQ required for each
device
Only one IRQ required for
entire chain
Generally backward-compatible Generally backward-compatible
Only one IDE drive on an IDE
channel can be busy at a time
Two or more devices can
operate simultaneously
33

34. More SCSI Considerations

Drives perform better in RAID array

Devices are generally of higher quality and
more expensive

Hard drive with supporting host adapter and
cable costs more than an IDE hard drive
34

35. More SCSI Considerations
(continued)

Subsystem provides faster data transfer

Generally provides better performance


Often used on high-demand servers
A good SCSI host adapter allows connection
of other SCSI devices to it
35

36. Basic Steps for Installing SCSI
Devices
1.
2.
3.
Set jumpers or switches on host adapter;
install it on motherboard; install host adapter
drivers
Assign SCSI ID with jumpers or switches
Attach cabling to host adapter and each
device
36

37. Basic Steps for Installing SCSI
Devices (continued)
4. Verify termination at both ends of SCSI chain
5. Power up one device at a time
6. Install drivers and software needed to
interface between SCSI subsystem and OS
37

38. Installing a Host Adapter Card

Install card in expansion slot

Change/verify default settings (setup program)

Verify settings (Advanced Configuration
Options)
38

39. Installing a Host Adapter Card
(continued)

Windows supports host adapter

Loads device drivers automatically and installs
host adapter

Verify correct installation of host adapter
(Device Manager)

Install external SCSI device
39

40. Installing an External SCSI
Device

Install software to run device (includes driver)

Plug SCSI cable into host adapter port

Plug other end of cable into device

Set SCSI ID and install terminator on device

Connect device’s power cord to wall outlet;
turn on device

Restart PC and test device
40

41. Setting Device IDs During
Installation

Assigned automatically with SCAM-compliant
SCSI subsystem

Set manually without SCAM compliance; each
ID must be unique

For narrow SCSI, use IDs 0-7

For wide SCSI, use IDs 0-15

Host adapter usually has ID 7
41

42. Setting Device IDs During
Installation (continued)

For external device:


Push-button or rotary selector
For internal device:

Set of jumpers on the device
42

43. Setting IDs for External Devices
43

44. Setting IDs for Internal Devices
44

45. Installing a SCSI Hard Drive

Host adapter and cables must be compatible
with SCSI drive

Configure SCSI host adapter and SCSI hard
drive to communicate with each other:
1.
Set SCSI IDs
2.
Disable/enable disk drive and hard drive controllers
3.
Check terminating resistors
4.
Run CMOS setup for a SCSI system
5.
Load SCSI device drivers
45

46. SCSI Hard Drives Connectors
46

47. General Tips for
Troubleshooting SCSI Devices

Keep notes and read documentation

Add one SCSI device at a time

Use good quality components

Limit cable length
47

48. Troubleshooting SCSI
Installation Problems

Turn on external devices first, then computer

Check all connections

Check termination (common cause of
problems)

Verify most updated BIOS

Install PCI host adapter in PCI bus that
supports bus mastering
48

49. Troubleshooting SCSI Boot
Problems

Confirm BIOS setup drive configuration is set
to “No Drives Installed”

Verify that SCSI drive is partitioned, has a
primary partition, and boot partition is set as
active

Back up SCSI hard drive and do low-level
format on it
49

50. Summary

How SCSI technology works to provide better
performance and greater expansion capabilities
for many internal and external devices

Advantages and disadvantages of SCSI

Installation and troubleshooting guidelines
50