by James R. Berry
James R. Berry is a free-lance writer specializing in making the computer industry understandable to the general public.
There's a growing universe of electronic services and information out there. And to plug into those conveniences you'll need a formidable gadget called a modem, your computer's eyes and ears to the world beyond your den.
Modems are necessary because computers can't send bytes over telephone lines. Bytes (groups of eight bits) travel through computerland like squads of eight soldiers marching abreast. A phone line acts like a turnstile, allowing only one bit through at a time. And here's where your modem (for modulate/demodulate) comes in.
The computer sends each byte to the modem one bit at a time. The modem assigns a certain tone to the "1" bits and another to the "0" bits. It then sends these tones over the line in rapid succession. And tones the telephone handles with aplomb.
The modem at the other end converts these tones back into their respective bits, and the computer reconstitutes this bit-by-bit stream into squads of bytes. The message that started off as bytes in one computer and became a stream of tones over the telephone line finally ends up as identical bytes (and identical message) in the receiving computer.
Two categories of modems, the acoustic and direct connect, accomplish this end in different ways. Acoustic modems have rubber cups that fit over a phone's ear and mouthpiece (the handset), which send and receive the audio pulses. Direct connect modems plug directly into a telephone's wall socket, bypassing the handset entirely. A length of telephone wire with a modular RJ-11 jack at each end does the job.
Of the two kinds of modems, you'll almost certainly want a direct connect. Acoustics are sometimes used by travelers to transmit from hotel rooms or pay phones. But other than such specialized cases, direct connects are no more expensive and are more reliable.
Understanding some modem terminology will ease your trip into the electronic world no matter what kind of unit you buy. First off is baud rate, a term derived from Mr. Emile Baudot, who invented the teleprinter code in 1874. This is the speed at which a modem sends data, varying between 50 and 9,600 (and more) bits per second.
For practical purposes, you'll only be interested in 300 and 1,200 baud modems, the popular standards. At 300 baud, text is delivered at about five words a second, a fast reading speed. A 1,200 baud transmission is roughly twenty words a second. At this rate, messages fairly whip across the screen.
Most modem communication today is done at 300 baud. In fact, the majority of computer bulletin boards (privately run, free data bases) talk only at 300. Commercial data bases have the capacity for both, though they charge more for 1,200 baud communication. Units with 1,200 baud capacity are roughly twice the price of 300 baud modems, and whether faster speed is worth added cost is one of those thorny personal decisions.
Another term you'll hear, and need to understand, when buying or using a modem is full/half duplex. Half duplex means communication that flows in only one direction at a time. CB radios and home intercoms are examples.
Full duplex means communications that can travel in both directions at once. Telephone conversations, for instance, are full duplex. Most modems feature full half duplex. Some only include full duplex. That's okay, since communications software can set a half duplex mode from full duplex. But those few modems that feature only half duplex capacity should be avoided. Most data bases need full duplex capacity to talk to your modem.
Years ago Ma Bell assigned certain tone frequencies to code the "1" and "0" bits and created another modem term: compatibility, which refers to the specific tone frequencies assigned to bits. "Bell 103 compatibility," for example, merely specifies tones that have become a standard for 300 baud modems. Compatibility of 1,200 baud units involves specific tone frequencies and a special coding method called phase shifting. Bell 212A (or Bell 212-the "A" is often dropped) is now the standard for 1,200 baud modems.
The term answer/originate mode steps in about here. When you place a call, you're in the originate mode. Your modem automatically sends out one set of tones and prepares to accept the other. Meanwhile, the answering modem (answer mode) gears up to receive your set of tones and send the other. If both modems are in the same mode, neither will send or receive the tones it expects. In this case the units will snarl briefly at each other, then break contact.
When transmitting to a friend, make sure you arrange who's in answer mode, who's in originate. Software or a modem switch takes care of this detail. Data bases and bulletin boards are in a perpetual answer mode. Call them in originate mode.
Getting your modem on-line with the world also involves dealing with a trio of related terms: stop and start bits, parity and bit length. Each eight-bit group (byte) that leaves your modem has an additional bit added to its beginning. This is a start bit, and it signals the start of a byte to the receiving modem. In addition, a bit is tagged onto the end of each group. This is a stop bit and signals the end of the byte. With these start and stop bits added, each transmitted byte is 10 bits long. Parity is a way of checking transmission accuracy. Any software above elementary level lists it as a protocol setting, which will read: "Odd"-"Even"-or "No Parity." (A protocol is an option you can usually select.) Parity checking (odd or even doesn't matter as long as both computers have the same setting) uses the eighth bit of a byte. And if you want parity checking, you've got to free up that eighth bit for parity's use.
You do this by another protocol setting, one called "bit length," or "word length." Set a bit (or word) length of "T' if you use parity checking. If you don't use parity, keep the bit length at "8." If the bit length set by two computers doesn't match, the message on your screen can resemble the random chatterings of illiterate monkeys.
So what does parity checking get you? That depends on software, which may command a repeat transmission of a faulty byte. Or it may flag the error with an asterisk or other mark. Or it may do nothing, which is so often the case that a "No Parity, 8-Bit" setting is commonly used.
One major hurdle to tapping into the electronic universe is the modem-to-computer cable link. This, seemingly, should be a simple problem. But you can count on the tooth fairy as much as most dealers for advice on matching modem to computer. Following is a rundown of what to ask for.
One of the few standards to emerge in the computer industry is the serial port into which your modem cable plugs. It's called the RS-232-C interface, with the "C" often dropped or the letters AIE sometimes added as a prefix. However: your computer's RS232 may be male or female. If male, it has tiny connector pins. If female, it has tiny holes into which the pins fit.
Modems are known as data communications equipment (DCE). The relevance of this for you is that a modem's RS232 interface is wired to receive data on pin #2 and to send it on pin #3. Your computer is known as data terminal equipment (DTE). Its RS232 interface should have its pins wired to complement those of the modem's cable. Some computers, perniciously, wire their interface as DCE. In this case, the modem's cable should be a DTE.
When it comes to specifying or buying a cable, your job is to make sure everything matches up: DTE-to-DCE wiring, male-to-female RS232 interfaces. How to go about this job? A quick look will tell you if your RS232 interface is male or female. Your computer manual will state (somewhere) if it's interface is wired as DTE (typical) or DCE (exceptional, but vital to know).
One more word about modems. You'll come across the term "smart" modem, which means these units have more capabilities than their less intelligent brethren. Smart modems feature such capabilities as automatic dialing and answering, repeat dialing, storage of phone numbers, automatic baud rate adjustment, and notification (by LED lights) of a host of factors such as carrier signal detect, 1,200 baud transmission, and data going out or in.
Some modems have loudspeakers that let you hear dialing tone and responses; others have clocks that time your call. Less expensive models are no-frills devices that simply connect, send and receive data.
A word about communications software: you've got to have a software program loaded for modems to work. Minimally, they set necessary protocols such as baud rate, activate your RS232 serial port and enable your computer to receive and transmit data. Depending on its complexity, software will store, edit and send data, echo the screen's contents to a printer and accomplish a host of other fancy things.
Communications software runs from about $40 to $300 and up. If you expect to be a heavy modem user, spending $100 or more for a program isn't out of line. But there's a lot of good public-domain (i.e., free for the asking) programming around. The people to see for these goodies are your local computer guru, members of a user group or savvy friends with modems.
|HEY, MR. COMPUTER-
I don't care exactly what a computer is. Here's what a computer should be: an entrance to a Black Hole.
Forget user-friendly. Make it user-spooky. A voodoo box. Duck, you sucker! We've got enough servile, straight boxes around: answer-phone, record player, video disk/cassette player, TV After punching all those buttons over and over and over, don't you feel the punch finger just itching for one more button-the button?
No, not a new neat little trick. Not more info, not more jive. This is about punching into the random factor, letting go the surprise in the circuit. Look: it's no accident that the computer screen mirrors the face of the user, darkly. So let it do that, exactly. Let this gizmo call us, each of us, by our secret name (the name we're sure nobody guessed). A box that's bad company. Wild box. Hot box. Pandora's box.
Now: like a Black Hole, how's that? One fun thing about a Black Hole: there's no way to tell precisely when you've slid into it. No known mathematical formula can calculate its rim. No borderline, no safety zone. Maybe you think: okay? still okay? not caught yet?-too late, gotcha! Put that in, not the physics' power, but the deep sinking hook of quasi-metaphysical/emotional longing, the undeniable gut pull ... put that into this box. Make it so it can zap us right out of reality.
Hey, Mr. Computer-Head, get us out of here! No more lifelike bathos: bubble memory, artificial intelligence, smart modems, machine logic, computer simulations.
Stimulate us. Connect, pierce, shake us with the good parts we forgot: mystery fears, unspoken desires. Meaning: tickle the spirit in the machine!
L. M. KIT CARSON, bicoastal journalist and screenwriter
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