The SCSI (Small Computer System Interface) bus is a parallel interface standard used to connect hard disks and other peripherals to computers. It uses an 8- or 16-bit wide data bus plus a parity bit for error detection, and 10 or more control signals. SCSI originally provided a bandwidth of 5 MB/s but newer extensions allow up to 320 MB/s. The SCSI controller arbitration policy prioritizes access to the highest priority device, but this can cause starvation issues for lower priority devices waiting long periods to access the bus.

1. Lesson 16:
Standard I/O bus─ SCSI
Chapter 11: Input/Output Organisation

2. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Objective
• Familiar with a standard I/O interface─ parallel
bus called SCSI
• Learn that SCSI data bus has data bus plus
parity check bit, has 8- or in recent extensions
16- or 32-bits communicate data as well as
address and 10 or in recent extensions more
control signals
• Learn that SCSI Communicate with the hard
disk with bandwidth of 5 MB/s, in recent
extensions 160 or 320 MB/s bandwidth

3. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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SCSI (Small Computer System Interface)
Bus

4. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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SCSI bus
• SCSI bus controller Communicate 8- or in recent
extensions 16- or 32-bits communicate data as
well as address and 10 or in recent extensions
more control signals
• Communicate with the hard disk with bandwidth
of 5 MB/s
• In recent extensions 160 or 320 MB/s bandwidth

5. Schaum’s Outline of Theory and Problems of Computer Architecture
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Bus protocols
• Allow any device on the bus to request the use
of the bus
• Have a policy for deciding which device gets to
use the bus if more than one device wants to
access it at the same time

6. Schaum’s Outline of Theory and Problems of Computer Architecture
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SCSI Data bus
• Data bus has data bus plus parity check bit
• Enables single bit-error detection in a data
transfer

7. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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SCSI bus signals

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SCSI ten Control Signals
• 1. RST—the reset signal activates to bring
all devices with SCSI controller to start up state.
• 2. SEL—when bus is used for selecting an
SCSI target (It differs from Poll count, instead of
poll count, the controller activates, SEL, the
SCSI controller examines SCSI ID and always
select the highest ID device
• Each SCSI device has a distinct bus request line
BRqi

9. Schaum’s Outline of Theory and Problems of Computer Architecture
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SCSI ten Control Signals
3. BSY (for Busy)—when SCSI bus is not
available to other device (controller)
4. ATN—activates when a device draws
attention before sending a message to a target

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SCSI ten Control Signals
5. REQ—like DRQ in sequence 1 Section
11.13 requests a data transfer by a device with
the SCSI controller
6. ACK—acknowledgement from the SCSI
controller that data transfer completes, like
DACK
7. MSG—when a message or information is
being transferred through the SCSI bus

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SCSI ten Control Signals
8. C/D—1 or 0 when a control (command or
status) or data word is being transferred,
respectively
9. I/O—1 or 0 when a data is input to SCSI
controller or output from the controller,
respectively
10. Parity

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High-speed SCSI versions
• Use double edge detection of the signals

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SCSI bus signal sequences

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Sequences of SCSI Signals
1. Sequence 0: Arbitration when Busy is not
active
2. Sequence 1: Selection using SEL—the
selection of signals is done

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Sequences of SCSI Signals
3. Sequence 2: Command or status or data
transfer
4. Sequence 3: Reselection

16. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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SCSI bus Arbitration policy

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SCSI controller arbitration policy
• Access to highest priority device first
• Advantage that it is easy to decide which device
gets access to the bus

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SCSI controller arbitration policy
Disadvantage
• Starvation
• Starvation when a device with a high SCSI ID
can prevent a device with a lower ID from ever
getting a chance to use the bus by making
repeated requests for the bus
• Can occur any time an arbitration policy always
gives one device priority over another

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Starvation problem
• Can occur any time an arbitration policy always
gives one device priority over another
• Some arbitration policies prevent starvation by
giving more priority to the devices that have
been waiting for the bus longest

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Summary

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We learnt
• Data bus has data bus plus parity check bit
• SCSI has 8- or in recent extensions 16- or 32-
bits communicate data as well as address and 10
or in recent extensions more control signals
• Communicate with the hard disk with bandwidth
of 5 MB/s, in recent extensions 160 or 320 MB/s
bandwidth
• Access to highest priority device first arbitration
policy

22. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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End of Lesson 16 on
Standard I/O bus─ SCSI