Classic Computer Magazine Archive COMPUTE! ISSUE 37 / JUNE 1983 / PAGE 36


Charles Brannon, Program Editor

You discover bugs (errors) in your 50-line BASIC program. If you could only see more than 20 lines at a time! You use your computer to keep track of your record collection, but realize that the valuable information on your TV screen is transient - erased when you "Press RETURN to Continue." You subscribe to a telecommunications network and despair when your hard-earned 300-baud text scrolls off the screen into oblivion. You may feel that you need a permanent record of your text, want to write (word process), or hang your artistic efforts on the wall.

Even though prices are falling, a printer is an investment. Your printer could cost more than your computer. After you've realized you need a printer, you need to decide how much printer power you require.


First you should identify your application. Do you need only paper listings of your programs? Simple "printouts" from your home financial program? Will you want to use your computer for word processing? Do you need to print on plain paper, on special forms, or to produce extra-wide accounting reports? Consider also the fact that as you expand your computer system, your needs could change. Does a certain printer have enough features to be satisfactory in the long run? Remember that most printers cannot be expanded as easily as computers can.

Printing Techniques

Printer manufacturers have come up with many ways to solve the problem of getting words onto paper. The most obvious method is similar to the way that typewriters print. A formed character "stamp," usually made of metal, is struck against a ribbon that leaves an impression on paper. On a typewriter, a series of levers controls which character hits the paper. IBM invented the "type ball," a hemisphere containing all the characters in various rows. To print a letter, the ball turns and tilts to select the right letter, then the whole ball strikes the ribbon.

It is not easy to interface a computer with a typewriter. First, typewriters are not built to tolerate continuous operation. Second, some of the interfaces are cumbersome and expensive, usually a series of solenoids that push the levers around, or a keyboard overlay with a separate solenoid for each key to mimic a human typist. The third problem is speed; you still can't drive the typewriter faster than about ten characters a second.

IBM Selectric ball

Close-up of an IBM Selectric ball. Some electronic typewriters can also function as computer printers with the proper interface.

The old-fashioned typewriter layout is prone to jamming; the type ball is too slow. Printer technology diverged to solve the problem. Dot-matrix printing was developed as an appropriately digital way of producing text. The daisy wheel was developed as a high-speed, reliable way of printing formed characters.

Daisy wheel print head

Some letter-quality printers are called "daisy wheel" printers because they use a print head shaped like a daisy with letters on the ends of the petals.

Thimble print head

Some letter-quality printers use a "thimble," a variation of the daisy wheel.

Dot Matrix

Take a look at the characters your computer displays on your TV screen. Each one is made of tiny dots, usually within an 8 x 8 box. Figure I shows a typical dot matrix representation of the letter "A". The dots are formed by a sweeping electron beam in your TV tube. Dot-matrix printers also use a series of dots to form characters.

The heart of a dot-matrix printer is the print head. Imagine it as a series of vertically stacked pins (see Figure 2). Each pin can be "fired" independently. To print a line of text, the print head sweeps across the paper. Characters are not formed "all at once," but one vertical line at a time.

There are several ways to print on the paper. Impact printers strike the pins against a ribbon to leave an impression on paper. Thermal printers use a column of tiny "spark plugs" to electrically vaporize (burn off) a special aluminum coating on thermal paper so that a black surface under the aluminum will show through. Ink-jet printers spray ink through tiny holes.

Each dot-matrix method has its advantages and disadvantages. Thermal printing is much quieter (and cheaper) than impact printing. Some thermal printers are virtually silent. Ink-jet printers are even quieter, but their price places them outside the home computer market for now. Impact printers, despite their sound and higher price, do not require special thermal paper. Another disadvantage of thermal paper is that it does not age well. It oxidizes over time and darkens appreciably.

Dot matrix print head

A print head from a dot-matrix printer. The ends of the tiny wires which form the characters are barely visible at the front-center of the print-head.

Friction Vs. Tractor

Also borrowed from typewriter technology, a platen is found in most friction-feed printers. The pressure of the rotating cylinder feeds single-sheet or roll paper. Unfortunately, friction-fed paper is subject to slippage and skewing. After ten forms, your printer may no longer be lined up with the paper. Skewing varies; some printers have excellent friction feed. However, the reliability of a tractor feed mechanism is generally superior for processing multiple forms. The "teeth" of a tractor feed mechanism fit into small holes on each side of the paper and pull it through the printer. Tractor feed is more expensive and requires continuous special pinfeed paper. However, the strips of holes can be detached and the paper separated into 81/2" x 11" sheets.

Tractor-fed printer

A tractor-fed printer with its toothed wheels and perforated paper. (The right-hand tractor is unlatched.)

Print Quality

Obviously, formed character printers (daisy wheels) produce the best-looking text, indistinguishable from a typewriter. This is important for business, where the "computery" style of dot-matrix is often unacceptable for letters. Some dot matrix printers, however, have exceedingly fine print quality. This is called correspondence quality, and can approach a typewriter's quality.

Correspondence quality printers space the dots closer together. A 5 x 7 character matrix is standard on average dot-matrix printers, but correspondence quality printers use 9 x 9, 9 x 14 (or 9 x n, where n can vary), or even higher densities.

Related to dot density are lowercase descenders, the "tails" on the letters g, j, p, q, and y. A 5 x 7 matrix is not really adequate for good descenders, which should extend at least two dots below normal letters. See Figure 3 for an example.

Descenders can make text easier to read, and if you do a lot of proofreading or copy-editing, this can make a difference.

Character Sets

Every printer has a slightly different typeface, or character set. The character set is the various patterns that the print head uses to form a character. Some printers use a simplified character set for high-speed draft printing. The patterns are less dense. However, that same printer might also offer high-quality dot-matrix printing, but at a lower speed. A printer should have upper- and lowercase, and all the normal symbols found on your keyboard. Some printers have special line-drawing graphics characters or foreign language symbols. Compare sample printouts from several printers when making your choice.


Dot matrix printers can also perform special tricks with the characters. They can elongate (print twice as wide) and condense (twice as small). Some printers can automatically underline text, others have horizontal or vertical tabs, or can double-strike or print in boldface. Printers can also vary the height of a printed line. Most printers are able to form feed (automatically advance to the top of the next page). This is done by sending a special message from your computer to the printer.


In addition to printing predetermined patterns (ordinary numbers, letters, etc.), some printers also let you control the print head directly to create your own custom graphics. You can create special symbols. And you can "dump" (copy) graphics from your computer, if it has a high-resolution screen. High-resolution is measured in dots-per-inch or dots-per-line. You may not need more resolution than your computer has. If your computer can display 320 dots per line, you won't necessarily need a printer's capability to print 960 dots per line. One caveat of high resolution is speed; the more tiny dots you have to control, the more data needs to be sent. You may require a special machine language routine to effectively use dot graphics, so check what's available before buying.

Printer Software

Look over the available software which is compatible with the printer you're considering purchasing. If your favorite word processor doesn't support the "FastPrint 110," you may not be able to use some of its features. However, some word processors let you embed special characters in your text to control any printer.


Generally, dot-matrix printers are fast - 60 to 80 characters per second (CPS). Letter quality printers (daisywheels) run from 25 to 50 CPS. Some low cost daisywheels print at around 10-14 characters per second, so consider your patience threshold before you buy. Several tricks are used to increase throughput. Time normally wasted during the carriage return, when the print head has to travel from the end of a line back to the beginning for the next line, can be used by printing backwards on the return. This is bidirectional printing, and can speed up overall output significantly.

Another trick used in conjunction with bidirectional printing is logic-seeking. A logic-seeking printer attempts to print the shortest line. If necessary, the print head may return at high speed to print a short line. Some printers, especially daisy wheels, also boast space skipping. This is most useful when the printer fills in blanks on a pre-printed form. Instead of advancing at a uniform rate, the printer counts all spaces received, and then quickly jumps directly to the next non-space printing location.

Random Access Printer

To free up the computer while the printer is zipping away, some printers contain an internal buffer. A buffer is just RAM memory, like your computer's RAM, but is inside the printer. The buffer accepts characters from your computer as fast as the computer can send them. It then feeds the characters to the printer at a more leisurely pace. If the buffer is large enough, your computer will be available almost instantly for other non-printer tasks while the printer is printing. Almost all printers have a one-line buffer, but some printers have 4K, 16K, or even 64K buffers! Also, some manufacturers market add-on buffers that attach between your computer and the printer.

Dot matrix, print head, descenders

Sample type styles

The Interface

Most computer manufacturers have a line of "official" printers. These are usually relabeled versions of other commonly available printers. Sometimes, the company that made your computer customizes the printer to add extra functions, or just a built-in interface. If you are buying a printer from a third party vendor, you may need a special cable or interface.

The two most common printer interfaces are Centronics parallel and RS232C serial. Many home computers use a proprietary (non-standard) input/output port and special cables. This does not lock you out from other printers if an "interface box" is available. You plug your computer's special cable into this "black box" and run a cable from the interface to the printer. Some interfaces also perform intelligent tasks, such as code translation, if your computer does not use the standard ASCII code (a convention for sending text as numbers).

Print Width

Most printers can print 80 columns per line, although some can squeeze in condensed characters in 132 character lines. Some printers have an extra-wide carriage. There are also 40-column and 32-column printers. If you plan to do word processing, you'll need an 80-column printer, since it matches normal 81/2" x 11" paper. Forty-column printers aren't as common as they once were, since 80-column printers have become less expensive. If you are interested in screen dumps and program listings, you need a width equal to your computer's display width (22, 32, or 40 columns).

More On Daisy Wheels

A daisy wheel printer prints by rotating a small metal or plastic disk with characters on long spokes, or petals. A hammer strikes the petal against the ribbon and paper. A variation on the daisy is the "thimble," where the petals are curved up into a cup, with the hammer in the middle of the cup. There are more moving parts and more metal and plastic in a letter quality printer than in a dot matrix or thermal printer.

When you're shopping for a letter quality printer, see how many type styles are available. Also compare ribbon prices. Some daisy wheels can even superscript and subscript (roll the paper up or down half a line), type in boldface or "shadow," or even plot graphics using a period as a dot. Some lower price daisy wheels can mimic the features of their "big brothers," using the same daisy wheel elements and ribbons.

If you want to add the valuable features of word processing, or want convenient hard copy at any time, a relatively small investment in a printer can change your whole outlook on your computer system.