Unit 1 Computer Systems Topic 3 – Peripherals COMPUTING (Higher) Unit 1 Computer Systems Topic 3 – Peripherals

PERIPHERALS Systems Booklet 2- Pages 3 - 4 What is a Peripheral?   Any hardware device external to the processor or main memory is called a PERIPHERAL. These are used for Input, (Keyboard, Bar Code Reader, Microphone, Webcam etc.) Output (Printers, VDU’s, LCD Panels, etc.) and Backing storage (Magnetic Tape, Optical Disk, Solid State Storage, etc.) Their ‘job’ is to connect the processor to the outside world. However, most peripheral devices are designed to be general-purpose, in that their design takes no account of which processor they may be connected to.  

Buffers and spooling. (look at later) Peripherals also work at much slower speeds than the CPU. This is helped by Buffers and spooling. (look at later) Sound cards can have their own processor and RAM. Video cards have their own own processor and RAM. As peripherals are processor independent, the signals from them are not always compatible with these peripherals, and in these cases an INTERFACE is required. What is an Interface?   An interface is the unit, hardware or software, which allows two pieces of equipment that would otherwise be incompatible to communicate with each other. Interfaces allow for differences in speed and data between peripherals and the processor.

The interface is responsible for What does it do?   The interface is responsible for Data Conversion - changing data from the processor form to the peripheral form (and vice versa) Data Storage - providing temporary storage of data in transit. Status Information - supplying information to the CPU concerning the readiness of peripherals. Control Signals - receiving and generating signals for the control of the peripheral. Device Selection - find out which peripheral is talking to the processor, or select as necessary

  Voltage – different voltage levels between peripheral & computer need to be ironed out. Protocols – rules that govern transmission of data. E.g. no of bits per packet, voltage levels etc. Speed - Different devices send and receive data at different rates. The devices agree a rate prior to transmission by utilising a protocol.

In some cases parallel to serial conversion is required Some devices are serial, e.g.1 bit at a time is transferred. Serial used for long (over 2m) distances. Some are parallel (printers), e.g.8 bits at a time. Used for short distances Problems with skewing – loss of data integrity. Serial can be slow but current use of fibre-optic cable makes it very fast.

Buffers Since modern processors work at very high speeds, one of the main problems associated with peripherals is, as said previously, speed incompatibility between themselves and processors. To speed up processing, most computers have specialised high-speed areas called buffers. An input buffer is a waiting area holding blocks of data transferred from a peripheral. An output buffer stores data already processed by the CPU awaiting output (for example to a printer).

Simultaneous Peripheral Operation On-Line (Spooling) This is another method of providing efficient use of the processor. Data is sent to a spool file on backing store until the output device is ready for it. (e.g. Background printing)

Memory Mapped I/O The physical layout of a particular memory system is called a Memory Map and it shows the organisation of memory. Most processors use part of their main memory to access interfaces and if the processor wants to send data to a peripheral it writes the data to the location specific to that peripheral, i.e. its address. Data can also be read from that same location.   This is memory mapped I/O

Input & Output Peripherals Keyboards QWERTY keyboard has its roots in mechanical typewriters. The key layout is designed to slow down operators to avoid jamming the keys. Key press causes a code to be sent to the computer. Sent via serial coble to keyboard controller. Sent as ASCII Codes Modified Keyboards Used to alleviate Repetitive Strain Injury (RSI) Customised keypads can have more (or fewer) keys all programmable to suit particular situations. Adjustable split keyboard in 3 parts to allow flexibility.

Input & Output Peripherals Scanners Flat bed scanner allows for up to A4 size documents Document placed face downwards on glass panel and scanned. Light beam reflects light from the document and photocells measure the light reflected. Analogue data needs converted to digital (A DC) Modern scanners use high bit depths to allow high resolutions. Images must be matched to their purpose No point in scanning at a resolution of more than 75 dpi for a screen based display. No point in scanning at 600 dpi for a printer rated at 300 dpi.

Input & Output Peripherals Scanners Accuracy – measured by how close the image is to the original. Resolution is the dots per inch (dpi) that can be detected by the scanner hardware. A 600 dpi scanner has 600 photocells per linear inch. Bit depth usually 24 bits Capacity Little internal buffering, rely on techniques to transfer the data. Storage can be high e.g. A4 page at 600 dpi requires 33.28MB for 8 bit and around 100MB for full colour. Cost Dropped dramatically in recent years Bundled software often the major selling point.

Input & Output Peripherals Sound Naturally Occurring Sound Natural sound is analogue in form To input sound to a computer Software samples the incoming signal Coverts the signal into digital form Usually compresses the file This is called ADC – Analogue to Digital Conversion Simplest input device is a microphone with sound card but sound files can be taken from a CD and downloaded from the Internet. Sound card performs the ADC and compression

Input & Output Peripherals Sound sampling Sampler listens to sound repeatedly and stores a number representing the amplitude each time Sampling Rate No of times per second sampler listens to the sound e.g. 22 kHz is 22,000 times a second Sample Size No of bits stored per sample e.g. 8 or 16 bit samples Compression Reduce storage space and reduce quality

Input & Output Peripherals Sound Accuracy Resolution – Three sampling resolutions in common use. 11.025 KHz (8-bit) – voice quality 22.05 KHz (8-bit) – Quality of AM radio 44.1 KHz (16-bit) – CD quality stereo [data sampled 44,100 times per second] Bit-Depth 8-bit sample size can hold 256 amplitudes per sample 16-bit sample size can hold 65,536 amplitudes per sample Capacity No built-in cache. Depends on fast access via the sound card to hard disk storage. 10.09MB to store 2 mins stereo audio. Compression required e.g. reduce sample rate / size or use a compression technique to reduce file size.

Input & Output Peripherals Video Video Digitising is performed by special video digitising circuitry installed on the motherboard of the computer. After market video cards can be purchased to enhance video. File Formats Quick Time MPEG AVi

Input & Output Peripherals Video To playback video on a standard computer it will need to be decompressed by hardware or software, usually on the video card. Standards AVI – (Audio Video Interleave) or Video for Windows. Being replaced by Active Movie which will playback AVI, QuickTime and MPEG. QuickTime – CODEC s/w developed by Apple but used by both Mac and PC. MPEG – Video board uses hardware to make compression much faster. Accuracy – Depends on Compression Technique, frame rate and resolution. Speed – Hardware must be fast enough to cope with stream of data to memory and to the hard disk. Cost – Not only card but good Multiscan Monitor required (17” and 19” nowadays)

Input & Output Peripherals Digital Camera Film replaced by an array of photosensitive cells. Images stored electronically using photosensitive diodes called charge coupled devices (CCDs) Intensity of light recorded in an image. Analogue values converted to digital using ADC Compression usually takes place. Bit map files turned into JPEG Image transferred to computer Serial Cable or USB “Floppy Disk” adapter Can then be printed, e:mailed etc.

Input & Output Peripherals Digital Camera Accuracy Resolution Measured in pixels or mega pixels – the more the better. E.g. 640 x 480 pixels or in megapixel mode 1280 x 960. Accuracy depends on the array of photosensitive cells.. The more sensors and the smaller they are the higher the resolution. Bit Depth Number of bits in proportional to the number of colours that can be represented. Capacity Based on resolution and memory in the device. Compression v altering resolution Cost Dropping as they become more common. Resolution main factor and also facilities (zoom, flash etc.).

Input & Output Peripherals Printers – Inkjet Printer Ink-jet Printers are based on one of three different types of technology: continuous flow ink-jet, liquid ink-jet or phase-change ink-jet. We will look at how a liquid ink-jet printer works. Liquid ink-jet or bubble-jet, operates by squirting tiny droplets of ink onto the page. The ink is first heated by passing an electric current through a coil. In milliseconds a bubble of vapour appears, forcing a tiny drop of ink from the nozzle onto the paper. Resolution is typically 600 to 1200 dots per inch, support the printing of text and graphics, colour and a range of shades. Speed is pretty slow with a range of 4 pages per minute to 15 pages per minute, depending upon the model.

Input & Output Peripherals Printers – Laser Printer This type of printer uses lasers to "write" a page image onto a special drum as an electrostatic charge. The charged drum attracts toner particles which are transferred to the page and heated to set the image. Usually a page is composed in the printer (often PostScript). Capacity On board RAM & processor needed to compose pages. The more RAM the higher quality graphics can be printed. Resolution 1200 dpi quite common (600 cheaper, 1200 expensive) Speed The faster in pages per minute (ppm) the dearer ranges from 30 ppm to 50 ppm with 20 ppm being about average.

Input & Output Peripherals Multiscan Monitor The CRT is the basis of most visual display technology. (slowly shifting) The screen is arranged as a series of lines of dots and each dot is made up of three small areas of red, green and blue called a triad. The intensity of light shone on each triad determines the actual colour of the pixel. The picture is redrawn between 50 and 100 times a second. This is the refresh rate. A monitor which operate at different refresh rates is known as a multiscan or multisync monitor. The refresh rate is controlled by the video adapter. Screen resolution is quantified by the dot pitch, the distance between the dots on the screen. Typically this is between 0.28 and 0.38mm, corresponding to 100 to 70 dpi.

Storage Devices Magnetic Magnetic storage devices include hard disks, floppy disks, Zip disks and magnetic tape. They are called magnetic storage devices because their recording surfaces are coated with a material that responds to magnetic fields to enable data to be stored. Storage devices can be fixed or removable. Removable storage devices allow the user to disconnect the device and physically transport data from one computer to another. Varieties of removable devices include the Iomega and Syquest hard disks and Jaz cartridges.

Storage Devices Magnetic Disk All the sectors around the disk, equidistant from the centre, form a track. With multiple platters, the collection of tracks on each platter, equidistant from the spindle is called a cylinder. When data is to be read or written, the read and write heads are moved to the appropriate track, where they wait until the relevant sector spins past. Speed Rotational speed of hard disks has improved, from 3000 (rpm) of early disks, to current rotational speeds of 5,400 and even 7,200 rpm. Performance is also measured in terms of the rate of data transfer from the disk. SCSI - transfer rate 5Mb/sec Ultra Fast SCSIIII transfer rates - 40 Mb/sec.

Storage Devices Magnetic Disk Capacity Hard disks have improved tremendously in their capacity to store data in the last 10 years. From the modest 10Mb disks of the early 80s to current 1+ Tbyte disks on many of today’s PCs. Access The hard disk is a direct access device, meaning that data can be directly read or written to any portion of the disk.

Storage Devices Magnetic Tape Storing data on tapes used to be the only solution to backing up large capacity hard disks. Now, with large, removable magnetic disks and optical CR-RW technology, this is no longer the case. However, removable storage media is comparatively expensive costing 10 times tape. Therefore tape is still widely used. Tape is read and written on a tape drive. Data is written to tape in blocks with inter-block gaps between them. A single operation writes each block Data is stored on magnetic tape as magnetised regions on the surface of the tape induced by the magnetic recording head. To read data, the tape passes under the read/write head and the stored magnetised regions produce very small voltages in the head, leading to a current in the head coil. This current can be analysed to give a representation of the stored binary data.

Storage Devices Magnetic Tape Capacity Magnetic tapes have large capacities, reaching up to several hundred gigabytes and come in a variety of sizes and formats. Since their introduction, tape drives have passed through many stages of improvement with extremely reliable Digital Audio Tape (44.1 kHz, 16-bit record and playback DAT) drives representing the current state of the art. A 4mm DAT tape can now store up to 240 Gbytes of data! Access Tapes are sequential access devices. Accessing data on tapes is therefore much slower than accessing data on disks. They are not suitable as storage media for applications where data needs be used regularly - where a disk is a more appropriate medium. Because tapes are so slow, they are generally used only for long-term storage and backup.

Storage Devices Optical Storage CD-ROM A plastic disk is scanned using a laser. It reflects off pits on the surface differently from lands (bumps) Re-writeable CD-ROM is now very common with re-writeable DVD’s also now available. Capacity – About 650Mb Speed – from single (150KB/sec) to 32x (or even 40x). The x refers to the times faster than CD Audio. Cost – CD-ROM Drives fairly cheap . Access – Always random

Storage Devices Magneto Optical Storage Based on a combination of magnetic and optical technologies. Active layer is magnetic material. Recording – magnetic material heated beyond a particular temperature by laser, allows magnetisation to be reversed. Reading – laser operates at much lower temp and reflected beam rotated by magnetic field and detected by read head. Capacity – 3.5” disks of 128, 230 and 384 Mb Speed – Varies as multiple of standard single speed Cost – decreasing all the time as different formats and capacities become available.

Storage Devices Solid State Storage Devices (SSSD) Solid-state storage devices are made up entirely from electronic components i.e. they have no moving parts. Also called RAM disks, as they take the place of a magnetic disk as a mass storage device. They can be in the form of a plug-in card or cartridge containing memory chips. The chips of a SSSD are typically static RAM or Electrically Erasable Programmable ROM (EEPROM or Flash EPROM). SSSD are used with devices where space is at a premium e.g. in a camera, or when portability is desirable e.g a USB flash drive.

Selecting hardware to match operational requirements When given a scenario like setting up a System you have to consider:- RAM requirements Memory must be enough to run the software and support all the data in the system. Backing Storage Big enough to hold the O/S, Applications and data. Processor Performance Usually as fast as you can afford but must be fast enough to support all the applications recommended. Peripherals Specify type of printer, monitors etc. Communications Attached to a network or set up a new network. Internet connection (broadband?)