In 1983, Tandy Corp. introduced the first lap-top computer, the Radio Shack Model 100. It was no larger than a school notebook and weighed less than five pounds. With word-processing software and a modern built in, it was a huge hit with journalists covering the 1984 president campaigns.

Aboard candidates' press plans, the sound of clattering portable typewriters gave way to the muffled clicking of Model 100 keyboards. Stories could be written on the fly and filed from the first telephone right into the home office's mainframe or minicomputer. No more dictation.

Four years later, the market is flooded with far more powerful lap-tops, some with much larger and clearer screens, faster moderns and greater storage. These new machine run all the top software written for the IBM family of desk-top computers.

With all this choose from, my choice of a computer for a traveling journalist is still the Radio Shack Model 100, or its successor model, the slightly lighter and thinner Tandy 102. The current list price is $499, about half the price of the least expensive IBM-compatible lap-top.

The Model 100 has all sorts of drawbacks. The liquid crystal screen is only eight lines by 40 characters, and it is hardly bright. But the Model 100 can usually be adjusted for readability in all but the dimmest light. Tandy's Model 200, which has a 16-line screen, is not nearly as adjustable and seems hard to read in almost any light.

The Model 100 has only 24 kilobytes of memory, expandable to 32. That's not much by today's standards, but it's enough for almost any news story. And Tandy makes a $200 external disk drive that uses 3 1/2-inch floppies holding up to 200K each. The disk drive can be kept in a traveling journalist's suitcase for use at night while the computer goes with the reporter throughout the day.

At less than five pounds, the Model 100 is easy to tote on and off buses and airplanes. However, its processing speed is slow and, after using a desk-top computer, it can be maddening to wait for the 100 to execute what your office machine would have done instantaneously.

Still, the 100 makes up for its lack of processing speed with blazing start-up speed. You turn it on and move a cursor over the name of the file you want to work on. Hit the enter key and you're in your word procesor and the file you've chosen is on the screen.

With the typical IBM-compatible portable, you turn iton and wait while the disk drive grinds away loading the operating software. Then you wait for it to load your word procesor and then wait for your word processor to load the file you want to work on. If you whipped it out to take a few notes while a candidate was talking, he'd be finished while you were still booting up your system.

There's only one IBM-compatible portable that gives the Model 100 serious competition for writing on the go. That's the Toshiba 1000, which at 6.4 pounds, with operating software built in, is light and quick-starting. But its 25-line screen is tiny, making the characters quite difficult to read.

If the Model 100 doesn't have enough features for you, third-party companies, led by Traveling Software of Bothell, Wash., (800-343-8080), offer a variety of ingenious enhancements.

THere is a tiny ROM (read-only-memory) chip that plugs into a slot on the 100's underside. It provides text-formatting power comparable to desk-top word processors, plus a built-in data base, an outline processor, and a clever feature that allows you to get more text on the screen by compressing the size of the characters.

The same firm also has a ROM chip with a spell checker called The Sardie, which contains the Random House pocket dictionary. But the ultimate enhancement for the Model 100 is not the Ultimate ROM II. It's Traveling Software's Booster-Pak, a chassis that fits onto the bottom of the computer, adding about an inch of thickness and 10 ounces of weight.

At $429, it costs almost as much as the computer, but it adds 136K of storage memory (expandable to 2 megaytes) and slots for several ROM chips like those mentioned above. It has built-in software to access the Tandy portable disk drive as well as software to use the extra storage RAM like a disk drive. The Booster Pak will also accommodate an internal Hayes-compatible 1,200-baud modem.

Other firms also make products for the Model 100, including Portable Computer Support Group of Dallas (214-351-0564). Its Write ROM offers text-formatting power similar to that of the Ultimate ROM II. PCSG also has a popular spreadsheet called Lucid. Write ROM and Lucid cost $99.95 each, but you can get both, plus an outline processor and a data base in PCSG's Super ROM, for $199.

The cost of such enhancements can make a Model 100 system as expensive as an IBM-compatible lap-top. But they don't add much weight, and low weight is the model 100's biggest selling point.

Brit Hume is a contributor to the Washington Post Writers Group. Humes is an ABC News Capitol Hill correspondent and the founding editor of a computer newsletter.