Why the IBM PC spawned a mob of look-alikes

In the microcomputer marketplace, one of the most important yet confusing issues has become compatibility. The IBM Personal Computer (PC) has become the de facto industry standard among personal computers (single-user machines in the $1,000-to-$10,000 price range, used in either home or office).

Last year, about 25 percent of the retail sales in this category went for computers with an IBM nameplate. Another 25 percent were the dozens of brands that claim various degrees of compatibility with the PC.

There are two potent reasons that manufacturers have built computers that resemble the IBM:

First, the nearly instantaneous success of the PC created a demand greater than IBM has been able to satisfy; so the makers of PC clones have rushed to capitalize on this situation.

Second, the software industry is now writing virtually all major programs for the PC; other brand machines that are truly compatible can also run these programs.

But, of course, competitors cannot simply duplicate the PC. This would infringe on the computer giant's patents, and IBM is known for protecting itself vigorously in the courts.

So the competition has faced the challenge of coming up with machines similar enough to the PC to run most of its software but not so similar that it invites legal action.

At the same time, IBM competitors have tried to add characteristics that distinguish their machines from the PC and its mob of look-alikes.

Some, such as Compaq, have concentrated on packaging. Compaq sells a very successful transportable computer, the Hyperion, which offers an amber screen and racy styling. Others, like the Seequa Chameleon, depend on a low-price strategy.

Still others have added features lacking in the IBM, such as superior graphics or quieter operation.

The net result has been the appearance of dozens of machines that are similar to the PC, but which have their own idiosyncrasies.

Adding further complications is an unfortunate tendency on the part of some companies to exaggerate the degree of PC compatibility their machines possess.

''There are a number of them which we call 'press-release compatible,' '' remarks Dr. Ron Ward of Future Computing Inc., an industry analyst who has studied this issue carefully and who presented some of his findings at a recent seminar sponsored by Future Compu-ting.

Future Computing Inc. has decided the best way to make sense of the current situation is to talk about four different levels of compatibility - (explained in accompa-nying box).

Within each of the levels there are significant differences among machines.

For instance, among ''operationally compatible'' machines, the Compaq, Columbia, and some others have adopted a hardware strategy that uses as many of the same components used in the PC as possible. This results in the highest degree of software compatibility.

Another approach, one seen in the Hyperion, is to employ some different parts but to compensate with special software.

According to Dr. Ward, this results in machines with ''fickle'' compatibility: It is nearly impossible to tell before testing which software will run correctly. In addition, the type of compatibility a prospective buyer will want depends on the use his machine will be put to, Ward notes.

For instance, a person shopping for a computer that offers word-processing and office accounting will want one that supports the IBM high-resolution mono-chrome display, while someone looking for a general-purpose home computer will likely be more interested in the capacity to run programs that use PC color graphics and sound-generation. Levels of compatibility with the IBM PC * Operationally compatible

These machines run nearly all IBM PC-labeled software right out of the box. They also have keyboards and display screens very similar to the PC's. They may be capable of using PC add-on boards, which provide such functions as clock/calendars, telephone communications devices, and additional memory. * Functionally compatible

These computers, while very similar to the PC, have enough differences that they cannot run every one of the important IBM programs. But the companies that make them carry enough clout to have the top programs rewritten for them. While the machines can read PC diskettes, they cannot use PC boards. They have screens and keyboards that are different from the PC's. * Data compatible

These computers can't run all the major PC software but can exchange information with PCs by reading and writing to PC diskettes. * MSDOS compatible

MSDOS (or PCDOS) is the PC's operating system, the program that does many of a computer system's basic housekeeping chores. Machines that use the same operating system, but cannot read PC diskettes or run PC-labeled software, fall into this lowest level of compatibility.