Peripherals: Output devices- printers

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Output devices- printers

Computer printers are used to produce hard-copy of com­puter output, normally data or text, but also, in the case of certain printers, graphics (i.e. image, such as drawings or charts). Printers vary in their capabilities so far as text enhancements (such as underlining or emboldening) are concerned, and also in their ability to print graphics.

Most printers are designed to receive data 'in parallel' from the computer, 8 bits at a time. These have to be connected to the parallel port (socket) on the computer, also called the Centronics port, and sometimes labelled LPTI (short for Line Printer 1). The connecting cable will contain a number of parallel wires, and may take the form of a ribbon cable, so-called because it is flat and wide, like a ribbon.

Some printers are designed to receive data 'in serial' from the computer, i.e. one bit at a time. These have to be connected to the serial port (socket) on the computer, also called the RS-232 port, and sometimes labelled COM1 (short for Communications 1). This is the port used by the computer to communicate with other computers, and I will say more about this in Chapter 8.

Computer printers also differ in other respects:

• There are character printers (which print one character at a time), line printers (which print a line at a time), and page printers (which print a page at a time).

• A variety of printing technologies are possible, the main ones being dot matrix, daisy wheel, and laser.

• Some printers can handle only text, whereas others can handle text and graphics.

• There are a variety of standards for control codes, which are commands sent from the computer to the printer to turn on effects such as underlining and emboldening.

However, most software packages are able to cope with these differences. They do so by means of printer drivers, special programs which adapt the output from the software to the printer. All you have to do is to select your printer's name from the list that is presented to you when you first use the software.

The main types of printer are described below. The main differences between them are:

• Their quality of output.

• Whether they are able to print graphics.

• Their print speed.

• Their purchase price.

Dot matrix printers

These are inexpensive, relatively fast, and versatile, and therefore very popular. The print mechanism consists of a matrix of tiny needles. By hitting selected needles so that they stand out from the rest, the printer is able to create the shape of a letter or other character, which is then transferred to the paper via an inked ribbon. The print speed is typically 100 characters per second or more, and these printers are able to produce double size or very small characters, or bold print, italics, or other effects. They are also able to print pictures or graphs, if used with graphics software.

However, the print quality is not all that high, since the characters are not perfectly formed but consist of a pattern of dots. The resolution of most dot matrix printers is around 100 to 150 dots per inch. To overcome this defect, many dot matrix printers offer a near-letter quality (NLQ) mode. When in this mode, the print head prints each line twice, the second pass slightly offset from the first, which has the effect of filling in the gaps between the holes and making the characters more perfectly formed. This gives results which, although not of the highest quality, are acceptable for most correspondence.

One of the earliest and most popular dot matrix printers for personal computers is the Epson FX-80. This has set a standard for control codes (for emboldening, graphics, etc.) which many other manufacturers of dot matrix printers have followed. Almost all software supports this standard, and nowadays you can buy an Epson-compatible for as little as £150.

A related type of printer is the line printer, used in the data processing departments of large organizations for high­ volume work. This is a much faster machine, and much more expensive. One version has up to 160 print wheels arranged in the form of a barrel, with characters embossed on each wheel, and able therefore to print up to 160 characters (a complete line) simultaneously. The quality is similar to that of the dot matrix printer, but no NLQ mode is provided.

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