For best route to speed, hitch a local bus to PC


Computer users simply want their computers to run faster.

They want their Microsoft Windows applications to be snappier, they want colorful graphics to pop onto their screens, and they want to wait less time for the hard disk to fetch data.

In characteristic fashion, the personal computer industry has sought to address those desires by proposing a series of new and incompatible technical standards, with all the requisite acronyms and initials: OPTi, VL or VESA and PCI, among others.

They fall under the general category of "local bus" technology.

Chances are that the next time you go shopping for a new computer, the salesperson will start bragging about the local bus. You may be tempted to take the express bus to get away from such jargon, but wait: It is important PC technology, and you will probably have to deal with it sooner or later.

Compaq, Dell, AST, Gateway, Digital, NCR and most of the other big computer companies have recently started adding local bus extensions to new machines. Even experienced users find the technology mystifying.

Basically, a local bus is a special kind of expansion slot inside the computer that lets certain peripheral devices -- mainly monitors, but also hard disks -- communicate directly with the computer's microprocessor, or central brain.

In most PCs, signals between the microprocessor and the monitor have to go through a device called a video adapter, and the video adapter is typically plugged into a slot on the main circuit board.

The slot itself is important because it determines the type of expansion cards that can be added to the computer and the speed at which data will travel between the peripheral device and the rest of the system. The data pathways are called the "bus."

If you have a computer that runs DOS or Windows, you probably have a bus called ISA, for Industry Standard Architecture. A few machines, mainly more expensive ones, have different kinds of buses, called EISA and MCA. Micro- processors change much faster than buses, which must remain constant from one generation to the next to insure compatibility.

The standard ISA bus on a PC is a bottleneck. Data passes through it at speeds of 8 million or 16 million cycles a second (8 megahertz to 16 megahertz). The catch is that newer microprocessors can churn through instructions at 25 MHz to 50 MHz, depending on the chip.

In an Intel i486DX-50, the chip operates at 50 MHz. The chips that control the computer's video functions are also faster than the bus's data transfer rate.

Wouldn't it be nice to bypass the slow bus and have the peripheral talk directly to the processor? That is the idea of a "local" bus, which is an extension to the standard ISA slot.

(On Macintosh computers, the same thing is accomplished on "processor direct" slots.)

So, when you buy a computer with local bus video, it means the video plumbing has been installed right next door to the main processor, connected by its own private, local, bus. There is no need for the video information to take the long route through the regular, slow bus.

The result is that all the information needed to create images on the computer monitor is being processed at speeds close to those of the microprocessor. Everything on the computer screen seems to move faster, and the user is happy.

As multimedia systems add full-motion video to a computer's repertoire, local bus will become even more important.

With a single local bus standard, you could buy any computer with local bus slots and add any local bus expansion card, regardless of who makes it.

As it stands now, you have to be very careful. Some computer makers are in favor of a standard called VESA, some are backing one called OPTi, some are going with an Intel Corp. proposal called PCI, and some are backing more than one, hedging their bets.

For now, the average PC user can relax and take advantage of whatever local bus technology is offered by his or her computer maker. The confusion over standards is of primary concern to computer and peripheral makers, software companies and the relatively few PC users who must plan far in advance.

(Peter Lewis works out of the New York Times' Austin, Texas, bureau: [512] 328-8258.)

Copyright © 2021, The Baltimore Sun, a Baltimore Sun Media Group publication | Place an Ad