Input/Output
What is I/O?
• How we get the CPU to communicate with
devices
• From the computer’s point of view, it’s just
1’s and 0’s
• Gets interpreted by the device it is sent to
(output)
• Computer interprets data received (input)
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Devices
What are devices?
Obvious ones…
• Mouse
• Keyboard
• Printer
• Modem
Not so obvious ones…
• Floppies
• CD-ROM
• Hard disc drives
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The Bus - Reminder
A collection of wires which carry data between the
CPU, memory and I/O devices
• Address Bus: carries memory address values from
the CPU to the memory and I/O
• Data Bus: carries all data, including machine
codes, in all directions
• Control Bus: a small number of wires carrying the
read/write and enable signals, and a few other
‘control’ signals
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I/O Devices
Address bus
Data bus
Control bus
I/O Port
Interface between computer’s
bus and I/O device
Device-specific
communication link
Peripheral
Actual I/O device
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Peripherals
These are the devices that actually do the physical
input, output and data storage operations.
Examples: keyboard
printer
mouse
floppy disk
VDU
modem
They do not connect directly to the computer’s bus.
A peripheral is anything connected to the computer
except the CPU, memory and power supply
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I/O Ports
These are the devices that provide interfaces between
the computer’s buses and the peripherals. In
general, they do the following:
• Recognise when they are being addressed, or
selected
• Provide at least one (but typically at least two)
addressable locations that can be read and/or
written like a memory location. These locations
are called I/O registers, and can be classified into
data registers and control registers
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I/O Ports
• Respond to commands from the CPU
– CPU writes to the control registers
• For input devices: transfer data from device
to input data registers
– CPU can then read data from these registers
• For output devices: transfer data from data
registers to device
– CPU will have already written data to registers
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I/O Ports
• Provide a variety of protocols for data transfer to
and from the peripheral, all under CPU control
• Provide status and error information to the CPU
e.g. ready, broken, run out of paper etc
• Issue interrupts to signal the occurrence of certain
events (e.g. data received), as specified by the
CPU (more later)
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Parallel and Serial Interfaces
I/O ports can be classified according to the
type of interface they implement. The two
commonest are:• parallel interface
(e.g. to a printer)
• serial interface
(e.g. to a VDU or modem)
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Parallel Interface (typical)
Think: pins
on a printer
cable before
USB!
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Example protocol: Handshake
RDY: sender says
data is ready
ACK: receiver
acknowledges receipt
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Receiver acknowledges
transmission over
Receiver acknowledges receipt
Sender stops asserting data ready
Sender asserts data ready
Sender puts data on lines
The Handshake
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Serial
Interface
(transmitter)
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The Serializer
The serializer takes the (8 bits of) input, and
translates them to a single stream of data
It tells the CPU (status and control register)
when data transmission is complete
It reads from the status & control register
information about how to format the data, and
what speed to send it
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USB
• A bit different…
• Four wires
– Two for data transmission
– One for ground
– One for voltage (+5V)
• Data sent in packets, addressed to destination
• Lower noise, fewer errors => faster data transfer
than traditional serial and parallel ports
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Firewire
• Like USB but better
– Nearly twice as fast (USB2.0=480Mb/s;
Firewire800=800Mb/s)
– Allows longer cables (USB2.0 max 4.5m; Firewire
100m
• And also more expensive
– Which is why, although Firewire has been around since
1995 (Apple created it), USB is more used
• For really fast connections (e.g. video) Firewire is
best; for others, it doesn’t matter
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Test Yourself!
1. Think of some peripherals. Do they use
serial or parallel interfaces?
2. What is an I/O port?
3. How does handshaking work?
4. Can you draw a timing diagram for
parallel interface handshaking?
5. What about the serial interface?
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