Classic Computer Magazine Archive COMPUTE! ISSUE 46 / MARCH 1984 / PAGE 24

All About Adding Peripherals

Ottis R. Cowper, Technical Editor

In the November 1983 issue we examined some of the factors to be considered when purchasing computer peripherals. This month, let's look at some add-ons available for home computers, how they work, and why you might want them.

Tape And Disk Drives

Mass storage devices—disk drives and tape recorders—are a necessity unless you're only planning to use cartridge software.

Tape units and disk drives really aren't luxury items—you've got to have one or the other, unless you're using only cartridge software or you're willing to type in a program every time you want to use the computer. The drives are called mass storage units because they allow you to store large amounts of information for later use.

Despite their different appearances, tape and disk drives use a similar technology. Both create and interpret specific magnetic patterns on a special recording medium, the same process used in audio recordings.

In a tape drive, the recording and reading head is fixed, and the tape moves past it. As the tape goes by, the head can either create a new magnetic pattern on the tape (record) or interpret the magnetic pattern currently on the tape (read). A tape drive is a sequential device; to get to the last program on a tape you have to wait for all the preceding programs to pass by.

The Head Moves In A Disk Drive

You may understand a disk drive better if you think of the diskette as a circular slab of recording tape, which is just what it is. In a disk drive, the head is not fixed, but can move back and forth across the surface of the disk. The advantage of this is that the disk head, unlike a tape head, doesn't have to wait for the desired part of the recording to come along. Instead, it can jump directly to the spot on the disk where the desired information is stored. The drawback is that the disk drive's moving head must be positioned very precisely, to within a small fraction of an inch. It is this added degree of precision that makes disk drives more expensive than tape units.

Which storage device you choose is to some extent a matter of personal preference and to some extent is determined by your planned application. If you're only planning to store programs and small amounts of data, and you feel that waiting a little longer for programs to load is a good trade-off for a much lower price, a tape unit will probably be sufficient. Using tape might also be a good idea if children are the primary users of the computer. Cassettes, unlike diskettes, are sturdy and can withstand quite a bit of abuse.

On the other hand, if you are handling large amounts of data or if you don't want to wait several minutes for long programs to load, a disk drive is the obvious choice. For word processing, most users choose the disk drive because of the volume of data which must be stored and retrieved.

Also, if you will be purchasing software on a regular basis, consider the format in which most software for your computer is available. For example, most commercial software for those computers which do not accept cartridges is available on disk, which would necessitate a disk drive.

Disk And Tape Alternatives

Alternatives to tape drives and disk drives are available. For example, there's the stringy floppy, or wafer tape drive. Stringy floppies store data on tape, but the tape is in a loop and operates much faster than standard cassette. These devices usually cost more than tape drives, but less than disk drives.

For the really high-volume user who must handle very large amounts of data at very high speeds, there is the hard disk. The technology is the same as for diskettes, except that the recording medium is bonded to a metal plate that rotates at very high speeds.

Most hard disks are permanently mounted in their drives, although a number of models have been recently introduced for which the disk is in a removable cartridge. Many diskettes would be required to hold the amount of data which can be stored on a single hard disk, and the data can be stored and retrieved much faster. However, hard disk drives tend to be quite expensive, so they are common only among the most serious home computer users.

Game Controllers

Despite all the claims of utilitarian applications, most home computers are at least occasionally used to play games. Almost all games require the player to control some sort of action on the screen. While the keyboard is useful for entering programs into the computer, you will, unless you have exceptionally nimble fingers, find it only marginally acceptable as a game controller. Fortunately, there is a wide range of alternatives.

The most common game controllers are the ubiquitous joysticks. These come in two basic types. In joysticks for the Atari, Commodore, TI, and Coleco computers, moving the handle of the joystick closes one or two of four switches. If only one switch is closed, one of the four horizontal or vertical directions has been selected (left, right, up, or down). Pulling the handle in one of the four diagonal directions causes two of the switches to close simultaneously. Thus, these joysticks allow you to select one of eight directions.

Game controllers come in a variety of styles, each of which has a different "feel." Shown here are paddles, a joystick, a trackball, and a pressure-sensitive controller.

Directional vs. Positional

If we call the previous type a directional joystick, then the type used on the Apple, TRS-80, and IBM is a positional joystick. Moving the handle changes the setting of a pair of variable resistors, one on the horizontal axis and one on the vertical axis. This has the effect of changing the voltage level of the joystick output.

Though the switches in a directional joystick can be read directly as a number by the computer, additional circuitry is required to measure the voltage levels from a positional joystick and to calculate an appropriate value for the horizontal and vertical (X,Y) position of the handle. In a typical positional joystick, holding the handle in the upper-left corner produces a reading of 0,0. The upper-right corner is 0,255; the lower-left 255,0; and the lower-right 255,255. Values for other positions fall somewhere in this range, the center being around 127,127.

Many varieties of both types of joysticks are available, and choosing among them is strictly a matter of taste. Some joysticks have huge hand-grips, some have slim handles, and others have knobs on top. Some have a push button on the base of the unit, others have one on the handle, and others offer you a choice of either. Some people prefer heavy joysticks with firm handle motion; others prefer lightweight models with handles that move freely. Before selecting a particular joystick, it's wise to go to your local computer products dealer and take a few "test drives."

The next most common type of game controller is the paddle. A paddle is essentially half of one of the positional joysticks described above. The knob on the paddle controls a variable resistance, hence a variable voltage, which is translated by computer hardware into a number that reflects how far left or right the paddle is turned. Paddles are generally used in those games which involve only left-right or up-down movement, but not both. Paddle controllers usually come in pairs, but are not available for all computers.

Some Specialized Controllers

The other types of game controllers tend to be highly specialized. For example, there is the trackball, which can be used in place of a directional joystick. Briefly, spinning the ball activates circuitry which produces the same effect as rapidly pushing the joystick in the direction that the ball is spun. Thus, the trackball can be a good replacement for a joystick in games that require rapid movement all over the screen.

Before you buy any game controller, take a minute to measure just how far from the computer you'd like to sit when you're playing a game. Then, when you pick out a controller, make sure its cord is long enough. Of course, if the one you want comes up short, joystick extension cords are available. Or, if you want to free yourself from all those wires, Atari has a joystick with no wires at all. A small radio transmitter, built into the base of the joystick, signals to a receiver which you plug into the computer's joystick port.


Printers are among the most popular peripheral devices, and most computer owners plan to add one sooner or later. Printers allow you to make permanent copies of program listings and output, as well as copies of screen displays and graphics patterns. As with all other peripherals, the available printers vary widely in price and quality.

A printer, among the most popular of peripheral devices, allows you to make permanent copies of program listings and program output.

There are three basic types of printers for home computers: thermal, dot matrix, and daisywheel. All function by accepting character codes and translating them into printer commands to place the image of the desired character on the page. They differ in how the character images are produced.

The printhead of a thermal printer consists of a horizontal or vertical row of small electrodes. As the printhead moves across the paper (or, depending on the printer, as the paper moves past the printhead), the electrodes burn a tiny dark spot in the specially coated surface of the paper. The printer creates the dots in patterns that form the various alphanumeric characters, just as characters are formed on the video screen by lighting up tiny dots.

The advantages of thermal printers are that they are quiet, durable, and inexpensive. The disadvantages are that the special paper required is more expensive and usually a bit more difficult to find than regular printer paper, and some types of the paper tend to turn dark with age.

Dot-Matrix Printers

The concept behind dot-matrix printers is similar to thermal printers—in both the printed characters are formed from patterns of dots. However, rather than burning the dots into special paper, the print-heads of dot-matrix printers have a vertical row of tiny wires or needles that strike an inked ribbon against standard paper. The number and size of the dots produced per character determine the printing quality.

The characters to be printed are formed within a grid, like the eight-by-eight grid used for designing characters on most home computer screens. A common arrangement is nine dots high by five dots wide. More dense arrangements allow for better character definition and hence better looking characters.

The daisywheel in printers of that type has the characters that can be printed arranged on the petals of the printwheel. It's as if someone picked all the letters out of a standard typewriter and arranged them in a circle. To print a character, the printer rotates the printwheel until the desired character petal is at the top, then strikes the petal against an inked ribbon just as in a regular type-writer.

Superior Print Quality

As might be expected, the printing quality of a daisywheel is also similar to that of a typewriter. Balanced against this superior print quality, daisywheel printers are both more expensive and, in general, slower than thermal or dot-matrix printers.

When deciding which type of printer to buy, consider how you will be using the printer. For example, if you simply need to whip out an occasional program listing, an inexpensive 40-column thermal or dot-matrix printer should suffice. If you wish to do word processing, you'll probably want a printer that can give 80 or more columns of output so that you can use standard paper. For casual writing or correspondence, a good quality dot-matrix printer is quite acceptable. However, a daisywheel printer is usually required for serious word processing which demands a professional, typewritten appearance. On the other hand, if you're interested in printing out charts and graphs or screen images, you'll need the graphics capabilities of the thermal or dot-matrix printer. In any case, try to see a sample of the printer's output before you buy so that the print quality won't come as a rude shock the first time you use it.

Serial And Parallel Interfaces

There are two methods of sending data to printers, serially (one bit at a time) and in parallel (eight bits at a time). Some computers allow printers to be plugged in directly, but most require special interfaces. You should choose the interface before, not after, you buy the printer, so that you can be assured of being able to connect the printer to the computer.

There's really no compelling reason to choose serial interfacing over parallel or vice versa, except that parallel printers are generally less expensive. Your computer could be a determining factor, since some computers have their built-in printer handling routines set up to prefer one method over the other.

Other types of printers are available. For example, there are dot-matrix printers which can print in a variety of colors. There are ink jet printers which produce exceptionally sharp characters on the page by spraying microscopic droplets of ink in carefully controlled patterns. There are even printers which use a laser to form characters which almost match the quality of typesetting. However, these printers are currently too expensive for most home users.


Modems (modulator/demodulators) are your computer's link to the outside world. They open to you the world of telecommunications, allowing your computer to exchange information with other computers.

A modem translates digital data from the computer into sound signals that can be transmitted over the phone lines, and converts sound signals from other modems back into digital data for the computer. There are two types: acoustic and direct connect. With an acoustic modem there is no direct connection between the computer and the phone line. The mouthpiece and earpiece on the phone handset fit into cups on the modem.

Modems let your computer exchange information with other computers via telephone lines. Direct connect modems plug directly into the phone line. With an acoustic modem, the telephone handset fits into a pair of soft cups.

A direct connect modem plugs into the phone line. Rather than creating audio signals, it impresses the equivalent electrical signals directly on the phone lines. This prevents the direct connect unit from picking up stray noises as data, which is sometimes a problem with acoustic modems. Plugging into the phone lines also makes possible several advanced features, such as auto-dial and auto-answer, whereby the computer can dial or answer the phone by itself. However, these features are found only on the more advanced direct connect modems.

Who Do You Want To Talk To?

To determine if you can use a modem, you must first ask yourself if there is anyone out there you and your computer want to talk to. There are several companies that specialize in providing tele-communications services to small computer owners, most notably CompuServe and The Source. Many modems come with information on accessing one or both of these services. In addition to the large companies, many computer clubs and user groups around the country maintain electronic bulletin boards that you can call for exchanging information, messages, and perhaps even programs.

If you have a friend with a computer and modem, you can exchange programs and messages directly between your computers. Keep in mind that unless there are a number of services you can use locally, you may find yourself running up excessive long distance charges to make use of your modem.

Memory Expanders

Memory expanders do just what they say: give you more memory for programming and storage. If you find yourself running into OUT OF MEMORY errors on a regular basis, or if you're planning an application which will involve the storage and manipulation of large amounts of data, you may want to consider purchasing one of these units.

Memory expanders give you additional work room within your computer. Expanders range from simple memory chips to complicated boards with cartridge slots and reset buttons.

On the other hand, if you aren't experiencing these problems, you probably don't need any additional memory, unless you're troubled by the fact that your neighbor has more kilobytes than you.

The simplest memory expanders just give you more of what you don't have enough of, RAM (Random Access, or read/write, Memory) chips, along with the circuitry to determine the addresses of the new memory locations. These are for computers which haven't yet reached their full memory capacity.

There are also more sophisticated memory expanders for computers which already have as much memory installed as their microprocessors can address. These use a special technique called bank switching, whereby blocks of memory can share the same addresses as long as only one of the blocks is in use at a time. The blocks of memory that are not switched in will still retain data until they are again selected. This switching, of course, requires extra circuitry and hence extra cost.

Some memory expander packages, especially those for the VIC, add extra features such as cartridge slots and reset buttons. Some memory expander cards for other computers allow you to buy the card with less than the maximum number of memory chips installed, so you don't have to pay for all the extra memory at once, but won't need another card when you purchase additional memory later. Try to buy a card that holds the greatest possible number of memory chips so that future expansion will take no additional card slots. These are the sorts of features that you should consider when shopping for memory expansion.

Additional Input Devices

A number of peripherals are available which make it easier to communicate with your computer. These include the numeric keypad, the light pen, the touch pad, and the mouse. All provide ways to get information into the computer without touching the keyboard.

Most home computers have keyboards like those of typewriters. This is fine for typing in text, but programming often involves entering lots of numbers. Here the typewriter keyboard fails, because having the number keys in a row across the top slows down your numeric typing. A numeric keypad is essentially a small second keyboard you plug into your computer. It has the number keys laid out in the familiar calculator pattern, with perhaps a few extra keys for additional functions. You'll need a program to allow your computer to read the keys. This add-on will be especially useful in financial applications where many figures must be entered.

The light pen is a device that lets you point to a location on the screen and have the computer know where you are pointing. To understand how it works, a short description of the TV display is in order. A video display is not a static picture. The image on the screen is actually flickering constantly, but at a rate of 60 times per second, which is too fast for your eyes to notice. An electron beam draws a series of stacked horizontal lines on the screen from top to bottom to form the display.

When you hold a light pen to the screen, the computer times how long it takes the beam to draw from its starting position at the upper left of the screen to the point where you're holding the pen. From this, the horizontal and vertical position of the pen on the screen can be calculated.

Using A Light Pen With Menus

Light pens are most often used with screen drawing routines. In fact, this is such a common application that many people don't realize that light pens can be used for anything except drawing on the screen. Even the name is somewhat misleading. Light pens are suited for any application which involves getting information on or off the screen. For example, a program in which the user must select an option from a menu on the screen could be set up so the user makes his selection by touching the light pen to the desired option instead of having to type in a letter or number to indicate the choice.

A touchpad is an input device consisting of a tablet with a square sensing surface. If you press down somewhere on the surface, the pad will provide a pair of values that represent the horizontal and vertical location of the point being pressed. The pad uses thin sheets of resistive film instead of a variable resistor, but the principle of operation is otherwise the same as that described for positional joysticks.

As with light pens, touchpads are most often used to create screen drawings. In fact, you've probably seen engineers on TV using very sophisticated touchpads and mainframe computers to do very complex designs like blueprints for cars or airplanes. The simple touchpads available for home computers can't match that, but the principle is the same.

The hottest new input peripheral on the computer scene is the mouse. A mouse is essentially a small trackball turned upside down. Mice (or mouses—the proper plural for the computer version has not yet been decided) are used to position a cursor on the screen. Instead of typing cursor control keys, you place your hand on the mouse and roll it up, down, left, or right on the table. The program using the mouse will cause the cursor to move accordingly. The mouse is supposed to be more user-friendly for beginners than the cursor keys, and mice are featured prominently in new software for such computers as the IBM PC and Apple Lisa. Experienced typists may find, however, that taking their hands off the keyboard to move the mouse is more distracting than using the keyboard cursor controls.

Light pens, touch tablets, and numeric keypads provide alter-native ways of entering data into your computer.

Exotic Additions

Speech synthesizers. Talking computers have long been a favorite of science fiction writers. Now it seems that their day has arrived. The production of speech by a computer is similar to the production of musical tones, except that instead of producing notes, the speech synthesizers produce phonemes, the sounds which make up basic units of human speech. By stringing these phonemes together, speech synthesizers can produce words. We've even seen packages that allow the computer to sing, play background music, and display a face which moves its mouth in synchronization with the song.

The limitations of speech synthesis in home computers relate largely to memory. The sounds of human phonemes are complex and require extensive programming to simulate. Also, the sound production capability of many of today's home computers is somewhat limited. Nevertheless, speech synthesizers are available now for most home computers, and as units with more memory and better sound become available, the use of the synthesizers should become more widespread.

Speech recognition units. These are the opposite of speech synthesizers. Instead of allowing the computer to speak, they allow the computer to understand spoken commands. Speech recognition is not yet as advanced as speech synthesis because there are so many subtleties to human speech. For example, regional dialects cause people to pronounce the same word in different ways. Then there is the problem of words with different meanings which are pronounced the same or nearly the same, such as for and four, eight and ate, etc.

However, speech recognition units are available for several computers, and even a version of BASIC called SpeechBASIC to accept spoken commands. Since so much of human communication is based on the spoken word, the ability to talk directly to your personal computer would certainly make it more personal. Watch for progress in this area.

Plotters.A plotter is essentially a mechanical drawing arm. It consists of a pen which can move horizontally and vertically across a drawing surface under computer control. By carefully controlling the pen's movements, detailed drawings can be created. Sophisticated plotters can even select from several different pens for multicolor artwork. If you're interested in producing graphics of a higher quality than is possible with a dot-matrix printer, you may want to investigate the variety of plotters available. Be forewarned that some serious programming may be necessary to get your computer to produce draftsman-quality work.

Coprocessors. Adding an additional microprocessor to your computer is like giving it a second brain. For example, Z-80 microprocessor add-on boards are available for the Apple and Commodore 64 to allow those computers access to the wide array of software written for the CP/M operating system. A special math processor is available for the IBM PC to increase the speed at which mathematical calculations can be performed.

Many other types of peripherals are available for today's home computers. For example, there are interfaces which allow your computer to turn the lights in your home off and on at programmed times, or to adjust your home thermostat. Other interfaces allow your computer to control video cassette recorders. There's even a peripheral to allow your computer to monitor the temperature, humidity, and barometric pressure to forecast the weather.

This continuing stream of new products shows that we're still far from reaching a limit to what can be connected to a home computer.