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Page 1

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Output Devices

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Printers
Factors affecting choice
­ Volume of output
· High volume require fast, heavy-duty printer
­ Quality of print required
­ Location of printer
­ Are multiple copies required?
­ Is colour required?

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Types of Printer
Dot Matrix
­ Impact Printer - Print head has 9 or 24 pins
­ Pins strike paper through a ribbon.
­ 24 pins give better print quality ­ dots closer
­ NLQ (Near Letter Quality) obtained by printing each line
twice, which second pass slightly displaced so…

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Types of Printer
Inkjet Printer
­ Very popular ­ often bundled with PC
­ Cheap with very good resolution, particularly on
special papers
­ Droplets of ink are fired at the paper
­ Large areas of colour may get the page too wet
unless special paper is used
­ Colour…

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Types of Printer
Laser Printer
­ More accessible now as prices come down
­ Similar process to photocopier with toner images
being fused onto the paper by heat and pressure
­ Very high quality
­ Virtually silent
­ Colour option can be very expensive

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Plotters
Used for high quality line drawings
­ Building plans
­ Circuit diagrams
Pen (Vector plotters)
­ Draw images using point-to-point data, moving pen over the
paper.
­ Low in price
Penless (Raster plotters)
­ Electrostatic, thermal or laser plotters
· Required for high-density images
· Maps
· Assemble drawings…

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VDU
Three attributes: size, colour and resolution.
It has its own RAM to store the image on screen
Amount of RAM will determine resolution and number of
colours that can be displayed.
Resolution
­ Number of pixels used to represent a full screen
Colours
­ Dependent on number of bits…

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Communicating with the CPU
Buses
­ Used to transfer data, addresses and control signals to various
components of the computer
Internal Bus
­ Connect the various registers and internal components of the CPU
External Bus
­ Connect the CPU to main memory and the I/O units
Interface Units
­ Because…

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Data Transmission ­ Parallel & Serial
­ A parallel bus with 8 lines can transfer 1 byte at a time
­ A serial bus only transfers 1 bit at a time.
­ Common serial interface is the 25-pin RS232C cable used to
connect an external modem to a PC
­…

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Buffering & Spooling
Buffers
­ Memory used to hold data during I/O transfers to and from I/O
devices
­ CPU operates much faster than printer so input and output have to
controlled independently
· Once I/O is initiated by CPU, a special I-O channel takes control.
­ Many printers will…

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