Saturday, March 9, 2013

Printers

Printers

There are a large number of different types of printer used for computer output. They differ in:

1 The order in which data is printed

(a) Character printers-characters are printed one by one moving across and down the page.

(b) Line printers-Print 'a line at a time'. Strictly the line is not printed all at once but it appears to be.

(c) Page printers-Print a page at a time.

2 Impact or nonimpact

An impact printer is one in which letters are formed by forcing the paper and the printing head together to print the characters. Either (a) the printing head is pressed on to the paper or (b) a hammer hits the paper on to the character shape. An impact printer requires an inked ribbon next to the paper.

A nonimpact printer is one in which characters are not formed by mechanical impact (e.g. an ink-jet printer).

3 How the characters are formed-the font

The most common types are:

(a) Matrix font-each character is made up of dots selected from a rectangular matrix.

(b) Solid fonts- the characters are produced using pieces of shaped metal (or plastic) as on an ordinary mechanical typewriter.

MAIN TYPES OF PRINTER

1 Page printers

The most common of these is the laser printer.

In the laser printer output is produced on a light-sensitive drum. The computer controls a laser beam, which is a very narrow beam of light. The laser beam is directed very rapidly over the drum, being switched repeatedly on and off. Ink particles are then fused on to the paper wherever the laser beam hit the drum.

Large laser printers use powerful lasers and can print up to 150 pages a minute. This type are used with large mainframe printers. Recently smaller laser printers have been introduced for use in small installations and in offices. These produce up to 20 pages a minute.

Advantages of laser printers

1 They are very fast.

2 They give very high quality printing.

3 They are quiet, because they are nonimpact.

Disadvantages of laser printers

1 They are expensive to buy.

2 They are expensive to run.

2 Line printers

(a) Ink-jet printers

Very fine ink jets are directed at the paper. The ink is charged and can be directed electrically. This is similar to the way a beam of electrons is directed to make up a television picture.

(b) 'Band' or 'train' printers

The character set is in a continuous loop of characters. Usually the complete character set is repeated several times round the loop.

The loop rotates in a horizontal line. There is a set of hammers across the paper. The hammer for each character position hits the paper and a carbon on to the right character as it goes past.

Advantages

Or ink-jet over band

1 They are faster.

2 They are quiet.

3 They give far better quality printing.

Of band over ink-jet

1 They are more reliable.

2 They are less expensive to buy.

3 They can print several copies at once using carbon paper between the copies. Or line printers over character printers

They are faster.

3 Character printers

(a) Dot matrix (impact type)

The paper is held on a cylindrical 'platen' with a ribbon held in front of it, much as on a typewriter. Each character is produced by a printing head hitting the ribbon on to the paper. The printing head has in it a rectangular matrix of needles (Fig 6), The appropriate needles are pushed forward electrically to form the shape of the letter required.

Printers

 

Fig 6 The five by seven dot-matrix pattern for the letter B.

(b) Solid font

The paper and ribbon are held as for the dot-matrix printer, but the character set is a set of shaped characters actually on the printing head. Examples of solid font printing heads are

golfball, where the printing head is spherical, and daisy wheel, where the printing head is a spoked wheel, made of metal or plastic and about 80 mm in diameter, with characters at the ends of the spokes (Fig 7).

Printers  2 

Fig 7 A daisy wheel

(c) Non-impact

Printers which rely on the paper and the print head hitting each other are noisy. Various methods have been developed of avoiding this, although at present they are not as popular as impact printers.

Advantages

Of non-impact over impact

They are very quiet.

Of impact over non-impact

They can produce multiple copies using carbon paper.

Of daisy wheel over dot matrix

1 The characters arc better formed.

2 They are quieter.

Of dot matrix over solid font

They are less expensive for a given speed.

Of character printers generally

They are cheap and reliable.

Disadvantages

Of daisy-wheel printers

The daisy wheels wear quickly.

Of character printers generally

They are far slower than page or line printers.

CHOICE OF PRINTER

The following are examples of the printing requirements of two different organizations and the printers which were chosen for them:

1 A large engineering laboratory where individual research teams each have a micro­computer. These computers also act as terminals to a mainframe computer on the site. This is used for printing listings of programs and large quantities of research data. The printing facilities in the main computer room are also used to produce research documents and reports. The computer room contains three printers:

(a) An impact line printer using a band system. This produces 1200 lines a minute. It is used for listings and research data where the quality is not too important.

(b) An ink-jet printer. This produces 20 pages a minute. It is used for important documents for high-quality printing.

(c) A dot-matrix character printer. This only prints at 60 characters a second. It is used to produce a computer room log. This log is a record of the programs which are run and of the resources which are used during the day.

2 A college which has two networks of microcomputers. One is in the business studies department. It is used mainly for word processing. The other is in the computing department. The following printers were chosen:

(a) The computing department has one dot-matrix printer. This prints at 100 characters a second for printing program listings. However, it can produce 'near letter quality' printing if used at half speed. It can also be used to do screen dumps of graphics if necessary to record students' projects. It has a continuous feed but the paper can be divided into separate sheets.

(b) The business studies department has a daisy-wheel printer for producing high-quality typing. It takes single sheets of paper.

(c) The daisy-wheel printer is rather slow. The business studies department also has a dot­matrix printer similar to that in the computing department. It does, however, have two important improvements:

(i) It will take single sheets.

(ii) It has a wider carriage so that it can take sheets of paper sideways.