Choosing a printer Types of Printers

Printing is the final stage in creating a document. That is the purpose of the printers joined to your computing equipment. Since the results you can obtain with different types of printers will vary substantially, here is a guide to help you decide which one is the most suitable for your needs.

To begin with, it must be taken into account that printers vary in cost, speed, print quality and other factors such as noise or compatibility. In fact, printing technology is evolving so quickly that there is always a printer for every application or personal requirement.

Daisywheel printers were very common a few years ago. They used a sort of wheel with solid characters which rotated and hammered against the ribbon, but they couldn’t print pictures or diagrams, and were very slow and noisy.

Dot-matrix printers use pins to print the dots required to shape a character. They print text and graphics and nowadays some of them can print up to 450 characters per second (cps), however, they produce relatively low resolution output – 72 or 144 dots per inch. The level of quality, while suitable for preliminary drafts, is not recommended for reports or books that have a wide audience. They are slower than laser printers but much cheaper.

One common type of non-impact printer is an ink-jet printer. It operates by projecting small ink droplets onto paper to form the required image. This type of printer is quite fast, silent and not so expensive as a laser printer. Nevertheless, you can expect high quality results because there are some ink-jet printers on the market with a very high resolution.

Laser printers produce output at great speed and with a very high resolution. They scan the image with a laser beam and transfer it to paper with a special ink powder. They are constantly being improved. In terms of speed and image quality they are preferred by experts for different reasons: they have a wider range of scalable fonts, they can emulate different language systems, they can produce graphics, and they have many other advantages. It goes without saying, that they are still expensive.

We must not forget to mention thermal printers. They use heat, a special kind of paper and electrosensitive methods. They are silent and are considered to be expensive. However, some colour models that emulate HP (Hewlett Packard) plotters cost too much to be included in the same category.

Photosetters can be regarded as an attractive alternative. They do not print on regular paper, but on photographic paper or microfilm. They can produce output with resolution of over 2000 dots per inch (dpi. In addition, they are extremely fast. Although hey produce the highest quality output, they have one important drawback: they are the most expensive.

Finally, plotters are a special kind of printer. Plotters use ink and fine pens held in a carriage to draw very detailed design on paper. They are used for construction plans, engineering drawings and other technical illustrations.

(E.C.U.)

Упражнение 44. Одним из устройств вывода является графопостроитель. Прочитайте текст и выясните его принцип действия. Затем прочитайте вопросы после текста и ответьте на них. Переведите текст на русский язык. При необходимости обратитесь к словарю.

The X-Y Plotter

An X-Y plotter is a device that moves a pen (one or more) across a normal piece of paper placed in the appropriate position on a board. Assuming that you have a good mechanical system then the resolution of the device can be as small as the thickness of the pen you are using. The basic idea is similar in principle to a gantry crane. The gantry moves along in the X-direction under the control of a motor. The pen then moves in the Y-direction up and down the gantry under the control of a second motor. To prevent the pen drawing all the time there is a device that will lift the pen off the paper when required. In some X-Y plotters an arrangement is made to select automatically several different coloured pens which are located in the pen park at the side of the plotter.

The principle of operation is quite simple. At switch-on it usually arranged for the plotter to go into a park position. This piece of paper may be put onto the plotter if desired. The usual mode of operation is then to print out to the plotter whatever appearing on the screen.

There will therefore be several major sections to the rest of this chapter: mechanical construction; electronic control; and software. Only the basic principles will be covered as the actual details will vary according to size of the plotter that you wish make and materials and tools that are available to you. However, there are some sections, e. g. pen mechanism and software, that will be transferable for most sizes of plotter.

End-stop detection

It is vital that provision be made to stop the plotter going too far in any direction. This can easily happen in the event of a fault in the software and especially whilst testing the hardware. Basically, all that is necessary is a microswitch placed at each corner of the plotter.

We now concentrate on the way in which the stepper motors will be controlled.

The motor drive routine will have a pair of coordinates (x,y) passed to it from the calling routine. It is the job of the motor drive routine to activate the correct stepper motors so that the pen goes in a straight line to its new destination (i. e. x,y). If we call the current position of the plotter (xnow/ynow), then the job of the routine is to move the pen from (xnow, ynow) to (x, y).

Suppose that we have to move 200 steps in the X direction and 100 steps in the Y direction. It is not enough simply to put the correct number of pulses out to each motor driver chip because then the Y motor would arrive before the X motor (assuming they were being driven at the same rate). We therefore need a method whereby one motor controls the other. For example, in the above case it would be possible to make the Y motor step every time the X motor did two steps.

This method is fine until we have to draw a line which is greater than 45° to the horizontal, i. e. we can't make the Y motor step every time the X motor does half a step. If the line is greater than 45° the Y motor takes over as the leader. If the line is exactly 45° then both motors step at the same time. In addition to this, if the line is horizontal then only the X motor need be moved, or if the line is vertical then only the Y motor need be moved.

1. Define an X-Y plotter.

2. If several coloured pens are to be used with this plotter, where are they to be situated?

3. Where is the usual parked position of the plotter?

4. One of the headings in this passage is ‘End-stop detection’. How would you explain this phrase in everyday English?

5. Explain the function of the motor drive routine.

6. When we are drawing a line:

a) which angle do the motors step in unison?

b) when does the Y motor move alone?

c) When does the X motor move alone?

d) when does the Y motor take the lead?

(c.S.)

Упражнение 45. Расскажите о любом устройстве вывода.

Упражнение 46.

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1. The product we are going to advertise is …

2. It is used for …

3. It is aimed at …

4. Its advantages over rival products are …

5. The slogan for our product is …