All About SCSI

1
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

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

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

5
SCSI Host Adapter
6
IDE versus SCSI Bus Communication
7
Host Adapters

• Manage all devices on SCSI bus
• Support internal and external SCSI devices
• Form a single daisy chain with devices

8
Considerations When Selecting a Host Adapter

• BIOS
• Expansion slot
• Bus mastering
• SCAM-compliant

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

10
Expansion Slot Considerations

• Host adapter must fit expansion slot(eg, 16-bit
  ISA, PCI)

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

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

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)

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

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)

16
Signaling Methods Used on SCSI Cables
17
Signaling Methods Used on SCSI Cables (continued)
18
SCSI Cables
19
Signaling Symbols
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

21
SCSI Connectors
22
Multiple Connectors on the SCSI Bus
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

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

25
External SCSI Terminator
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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

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

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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

29
SCIS Standards
30
SCIS Standards (continued)
31
SCIS Standard Cable Specs
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

33
Considerations When Choosing Between IDE and SCSI
Drives
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

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

36
Basic Steps for Installing SCSI Devices

• 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

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

38
Installing a Host Adapter Card

• Install card in expansion slot
• Change/verify default settings (setup program)
• Verify settings (Advanced Configuration Options)

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

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 devices power cord to wall outlet turn
  on device
• Restart PC and test device

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

42
Setting Device IDs During Installation (continued)

• For external device
• Push-button or rotary selector
• For internal device
• Set of jumpers on the device

43
Setting IDs for External Devices
44
Setting IDs for Internal Devices
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
• Set SCSI IDs
• Disable/enable disk drive and hard drive
  controllers
• Check terminating resistors
• Run CMOS setup for a SCSI system
• Load SCSI device drivers

46
SCSI Hard Drives Connectors
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

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

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

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