Classic Computer Magazine Archive ANTIC VOL. 2, NO. 2 / MAY 1983

Starting Line

Communications Concepts

by Bob Stewart Wherever you turn, whatever you read, it seems someone else is declaring The Computer Revolution. The information age is dawning, and we ATARI owners are part of it. Central to the information age is the distribution of information through computer communications. This article will explore some basic communications concepts.

Back about 10 years ago, most computer communication consisted of terminals talking to mainframe computers over ordinary telephone lines. Those terminals, now called dumb terminals, were little more than remote typewriters. If computer users wanted to exchange data, they usually did it with a tape. Today, in the midst of the microcomputer explosion, there are hundreds of thousands of computers with information that others could use.

You'd expect that communication would have advanced, too, but as far as most people are concerned, it hasn't The closest we usually come to true computer communication is through terminal emulation.

Terminal Emulation

We can run a program on our computer that will make it behave like a terminal. It can then talk to another computer over a telepnone line, with the remote computer acting as if it's talking to a terminal. As far as access to remote information goes, this works reasonably well for a lot of purposes. There is also a significant benefit to the fact that our computers, acting as intelligent terminals, can do some things that dumb terminals can't.

Before going further into some of the things your intelligent terminal can do, let's take a look at what's available in remote computers. There are two major services available today: bulletin boards and information services. Other services, such as using your computer as a remote bank teller, are coming, but are not yet widely available.

Many computer user groups and individuals offer free bulletin board services. These are computers that you can call on the phone to leave or look at messages. It's just like an ordinary bulletin board; all it costs is your telephone charges.

Information services are an extension of this same idea, based on the concept of timesharing. This allows many terminals to use a large computer at the same time. Commercial information services may be oriented to specific information, such as the Dow-Jones stock service, or general information, with news, buying guides, and other data bases that you can peruse. CompuServe and The SOURCE are examples of these. The general information services also offer such things as bulletin boards, live conversations with other users, and games. Usually you pay for the time that you are connected, plus your phone call.

Although this sounds pretty good, be warned that since the remote computer thinks you're a dumb terminal, it won't do anything fancy. You usually end up with a 40-column "glass teletype". Unless either your terminal emulation program or the remote computer is pretty smart, you get only those capabilities that all terminals have, which are few.

Smart Terminals

Now we'll look at the potential advantages of your intelligent terminal Which of these advantages it actually has depends on the terminal emulation program you use. Before you buy a terminal emulator, make sure it can do what you want.

The major advantage is that your intelligent terminal is really a generalpurpose computer. It will run all kinds of programs, You can use it to play Star Raiders, catalog your record collection, or write your own programs. If it was just a terminal, even an intelligent one, you probably wouldn't own it at all.

As an intelligent terminal, your computer's power stems from its ability to take input from, or direct output to, different peripherals. You can have it record output on printer, tape, or disk, as well as on the screen. Since your computer has its own memory, it can buffer data you receive and write it to a peripheral device after you disconnect. That means big savings in connect charges. You can even set up all your input beforehand and reduce the connection charges that would have been incurred to accomodate your typing speed and mistakes.

Up- & Down-Loading

Many bulletin boards and information systems offer programs that you can copy, or accept programs you want to offer. The process of copying such a program to your system from a remote computer is called down-loading. The reverse process is called up-loading. This process is subject to errors caused by line noise, so you must check the results when the transfer is complete.

Information passed between terminal emulator and computer must be in ASCII form (printable characters, like you get from a BASIC LIST command). Data in arbitrary binary form (as from a BASIC SAVE command) cannot be handled this way. Passing binary data requires more complex rules of conversation (a protocol), which can also do things like error correction. These more complex communication techniques are used in the coming wave of computer communication, the network.

That covers the software concepts we need, so let's discuss the hardware.


For your computer to talk over a telephone line, you need a modem. A modem modulates digital signals from a computer into audible, analog signals, puts them into a telephone line, and another modem demodulates them at the other end (hence the name, mo-dem). Modems transmit (talk) on one frequency and receive (listen) on another. Each modem must receive on the other modem's transmit frequency. If they both talk on the same one, the signals will ruin each other, and neither will hear it anyway. We therefore designate one modem as the originate side (usually the caller) and the other as the answer side (the callee). It doesn't matter which is which, as long as both agree.

A protocol that allows both sides to send at the same time is called fullduplex. If they must take turns, it is called half-duplex. In human terms, telephone conversations are full duplex and CB radio is half duplex. Again, both sides must use the same protocol.

Baud Rates

A modem's send and receive speeds are expressed as baud rates. For our purposes, you can think of this as bits per second. A 300 baud modem (the most common type) sends and receives at 300 bits/second, also called 30 characters (bytes) per second. You may notice that at eight bits per byte we lost a few bits of speed between 300 and 30. The modem uses a couple of bits per byte for purposes of its own.

An increasingly common transmission speed is 1200 baud. It's four times faster, but it's also more expensive.

Also, since both ends must agree on the speed, be sure your receiving party can also use 1200 baud before you spend extra money. If you can afford it, your best bet is a modem that supports either speed.


You need some way to attach the modem to your ATARI and to your phone. For this we have hardware interfaces. On the computer side, the most common is RS-232. RS-232~is a standard arrangement of pins and electrical signals that allows all kinds of different equipment to communicate. The ATARI doesn't have a built-in RS-232 compatible connector, so for any RS-232 modem you'll need an ATARI 850 Interface Module, which gives you four RS-232 connectors. You also need a cable with the right kind of connector at the other end (they don't always come with modems). There is also at least one modem, designed especially for the ATARI, that connects through the joystick ports. This works fine, but being non-standard, it requires-special software and is not compatible with software that expects RS-232.

On the phone side, there are two possibilities: acoustic coupled and d~rect connect. An acoustic-coupled modem has two rubber cups that fit a standard telephone handset. They can leak ndide, causing communication errors, and won't fit your modern, funny-shaped phone. Besides, you have to put the proper end of the handset in each cup, and if you get it wrong, you must redial she call. Direct connect is a much better method. This plugs directly into a telephone jack, either by itself or with your phone, through a Y-connector. If you're really flush, a direct connect modem with autodial doesn't even need a phone, On command tom the computer it will dial for you.

That about wraps it up. With these concepts, you are prepared to learn more about communication for your ATARI, and to make a start at deciding how you will share in the information age.