A computer system is made up from various components, some of which are inside the “system unit”, some of which are outside, and some of which are used to communicate with the user (see also User Interfaces, and Chapters 32 and 33 of Heathcote). Peripherals Peripherals are hardware devices attached to the computer system that expand the capabilities of the system. They are attached via an electrical connection (e. g. they are plugged into one of the ports at the back of the computer), and generally require some software running on the computer to control or “drive” them (see Drivers).
Examples of peripherals are things such as: Printers Plotters Scanners Barcode readers Storage Devices Computers need to store information, in order that it can be retrieved, searched, sorted, etc. There are two types of storage used by computers: Primary Storage: This is the computers “main memory”, or RAM (Random Access Memory). RAM is so-called because the information can be accessed by the processor in any order, unlike the information on, say, a tape, which always comes in sequence.
When we say that the computers in the school have 16Mb (mega-bytes) of memory, it is the primary storage we are talking about. The programs you are currently running, and in most cases the data they are using (e. g. your current document in Word) are stored in the primary store. Secondary Storage: The problem with RAM is that it is volatile. This means that is the computer is switched off or the power fails, then what is currently in the memory is lost (the word volatile means “evaporates quickly”). For this reason we need another form of storage; secondary storage.
Secondary storage is a permanent, non-volatile form of storage that usually involves a storage device recording the information onto some sort of physical medium. In the early days of computing, things such as punched cards, paper tape and magnetic drums were used, but modern examples are things like: Hard discs Floppy discs ZIP Drives CD-ROMs DAT or other data cartridges Secondary storage is usually much cheaper per byte than primary storage, which is convenient, because users often have a much greater need for it. My computer at home, for example, has 64Mb of RAM, and 6.
4Gb (1Gb is a “giga-byte”, or 1,000Mb) of secondary storage, and these two would have cost very roughly the same. Virtual Memory: As primary storage is usually more expensive than secondary storage, sometimes users don’t have as much as they require. A professional quality A4-size image, for example, would take up more than 64Mb on its own, without the operating system and the program used to process it. What some operating systems can do is operate a system called virtual memory. The memory manager can use secondary storage to save the information that won’t fit into the primary storage.
Because it’s unlikely that all of the data is used all of the time, the bits being used can be swapped between primary and secondary storage in a way that is transparent to the program. Input and Output Devices These are devices for communicating with the computer system. Input devices are used to receive input from the user, and include the following: Keyboard Mouse Graphics tablet Joystick Output devices are used to communicate information to the user in a form that he or she can understand, and include the following: Printers.
Plotters VDUs (Visual Display Units) You should be familiar with printers. Plotters produce also produce paper output (“hard copy”), but do so by “drawing” on the paper using pens; they are mainly used to produce technical drawings and plans (there is a fuller explanation on page 150 of Heathcote). Parallel and Serial Transmission Peripherals are usually connected to a PC via either a serial or parallel connector on the back. The data is sent by a different method through each. Through a parallel port, data is sent a byte at a time.
The parallel connector has eight wires, each of which will carry one bit of information, plus some other connections for control purposes (e. g. to indicate that the device is ready to receive, etc. ) Printers and some scanners are usually connected through the parallel port. In a serial connection, there is only one wire for the data (plus others for control), and the information is set a bit at a time. This is obviously slower than a parallel connection, but more reliable over longer distances. Serial communications can be either synchronous or asynchronous (see the Networking sections in Heathcote).
The serial ports on PCs use the asynchronous method in which there is no clock pulse. Further details about transmission to input and output devices (together with buffering and spooling) can be found in chapter 33 of the Heathcote book. Buffering and Spooling Peripherals often work at a much slower rate than the computer itself. To avoid everything grinding to a halt when you print, for example, the operating system or drivers can employ certain techniques. Buffering and spooling are both methods of using temporary storage for data, between the processor and the input or output device.
A buffer is usually an area of RAM, either in the computer itself, or in the peripheral device, where data can be stored before being processed, leaving the computer’s main processor and memory to get on with other things. Spooling works in a similar way except that the data is usually write to secondary storage, e. g. the disc. This is a common practice when printers are used on a network. When users print out their work, it is written to a file on the network server if the printer is busy, and printed out when the printer becomes available. Printers
There are various different types of printer, and which one you would select would depend on your requirements for: Printers fall into two main categories; impact printers and non-impact printers. As the names suggest, the former produce characters or graphics by striking the page, and the latter type use different methods. Dot matrix and daisy-wheel printers and both impact printers. Dot matrix printers are described in detail on page 148 of Heathcote.
Daisy wheel printers work like a type-writer; there is a plastic wheel with a character at the end of each “petal”. The wheel rotates to the correct position, and the letter, number or symbol is hammered onto a ribbon over the paper, thereby making an impression. Daisy wheel printers are limited to only one font (unless you change the wheel) and are not capable of producing text in different orientations (e. g. landscape), or graphics. The two most common types of printer are ink jet (or “Bubble Jet”) and laser printers. These are both non-impact methods.
Ink jet printers effectively spray the ink onto the page; this can make the output quite wet if large areas are covered (e. g. a picture is printed). They can also be quite fussy about the sort of paper used; if it is too absorbent, then the ink will spread like in blotting paper, and if the paper isn’t absorbent enough, then the ink will set on the surface and take longer to dry, therefore being prone to smudging. Laser printers don’t use ink at all, but a black powder called toner, which is melted onto the paper by a hot roller.
This means that you need to be careful when using transparencies and labels as they can melt (in the case of labels, the adhesive backing can melt and the labels can come off and stick on the roller). Laser printers are generally faster, but more expensive (especially if you want colour) than ink jet printers. Ink jet and laser printers are described in more detail in Heathcote, page 149. Display Devices The computer’s output is normally displayed on a monitor, or Visual Display Unit (VDU). For a PC to do this, it needs to have a video card installed inside it.
Other types of computer, such as the Acorn RISC machines have support built in. The size of the monitor is usually given in inches, measured diagonally from corner to corner (just as with a television). Common sizes are 14″, 15″ and 17″, although larger sizes are becoming more popular. When talking about the quality of the output, there are two attributes we need to consider: Resolution: the image on the screen is made up of a series of dots called pixels (picture elements); the higher the number of dots, the better the quality of the image.
The resolution describes the number of pixels that can be displayed on the screen in each direction (horizontal x vertical). The most common resolutions on PCs are: 640 x 480 800 x 600 1024 x 768 1152 x 864 1280 x 1024 These can be set in the Display section of Control Panel. To give you a comparison, the resolution of broadcast television is 625 lines. Colour Depth: the other thing to consider is how many colours can be displayed. The more colours, the more realistic photographic images will be.