Output Devices

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?

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 as to fill any spaces. Printers are often bi-directional Very versatile Colour possible via 4-colour ribbon – but quality not too good Can be very noisy

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 printing can be quite expensive

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

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 for machines

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 available for each pixel If only 1 bit, only 2 colours can by displayed 8 bits will allow 256 colours 16 bits (2 bytes) will allow 65,536 colours per pixel 800x600 with 65,536 colours requires approx 1Mb of video RAM

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 I/O units vary in terms of their speed, mode of operation and so on, they are not connected directly to the CPU. Each device will have its own interface unit

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 Most mouse interfaces used to be a 9-pin serial interface, although now they are usually a PS2 connection and increasingly USB (Universal Serial Bus connection) USB offers speeds of up to 12 megabit per second In serial mode only 1 line is used for data, the other lines are used for control signals, grounding etc Parallel is faster because 8 bits are transferred simultaneously. Parallel only available over short distances (2 – 3 metres)

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 have their own memory buffer Spoolers These speed up communication between devices which operate at different speeds Output for the printer, for example, may be spooled (written) to disk. When the printer becomes free, output will be printed This is the method employed on a network.

Installing Hardware Devices Scanners, printers, mouse, soundcard etc When these devices are installed on to a computer system they usually require a device driver to be installed. You may also need to assign a port for a printer Most devices can now be installed via Plug and Play on Windows systems. A printer driver carries information specific to the printer model – fonts and control sequences, etc. The driver will translate the font and formatting information in your document into a form that the printer can understand.