Local Bus to Pick Up High-End PC Users

With the coming of the new year, there is a new internal design that will gain popularity for use in the higher-performance personal computers equipped with Intel microprocessors.

Known as the PCI Local Bus, it is the fourth attempt in the last six years to improve upon the original IBM design for the AT class of PCs.

Computer design is pretty technical stuff, but you should understand enough about it to know the differences when you go shopping.

The engineers don’t make it easy, however, with terms like local bus , which comes from the electrical meaning of the word bus, not the vehicular meaning. In a computer, the bus is the main electrical circuit, or pathway, between the microprocessor and the various peripheral devices such as disk drives, video cards, modems and network cards.

IBM created the design standard for most of today’s personal computers in 1984 when it introduced the PC/AT. The microprocessor of that era was a 16-bit Intel 80286 chip running at a speed of 8 megahertz.

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Now, with the fastest 80486 and Pentium chips blasting forth at 66 megahertz, most computers are still built around an AT bus, still humming along at a leisurely 8 or 10 megahertz.

Building a better bus hasn’t been easy. IBM made the first effort, calling the result “micro-channel architecture” (MCA), which found favor mostly among its large corporate clients.

Compaq Computer led an anti-micro-channel campaign, joined by many of IBM’s competitors, with a design called EISA, for enhanced industry standard architecture. While many manufacturers offer high-performance EISA models, the design hasn’t supplanted the original AT, which is now called ISA, for industry standard architecture.

Last year, a new approach was taken: the VESA local bus, often abbreviated as VL bus, which began as an effort to meet the heavy video-processing demands of Windows software. (VESA stands for Video Electronics Standards Assn.)

Most manufacturers now have models with up to three (the maximum) VL slots on their mother boards, accommodating video, hard disk controller and network cards.

Now comes PCI (for peripheral component interconnect) local bus, offering these features to compete with the VL bus design:

* It allows up to 10 devices and has performance expandability in reserve for future requirements. Initially, the PCI local bus is a 32-bit pathway, running at 33 megahertz. Specifications allow for future expansion to 64 bits.

* It isn’t limited to systems with Intel microprocessors.

* Although originated by Intel, it was placed in the public domain and is now controlled by an organization of 225 computer industry members, the PCI Local Bus Special Interest Group. That is intended to assure that it will be a true standard, without proprietary considerations getting in the way.

* It coexists on the same mother board with other bus designs such as ISA, EISA or MCA (as does the VL bus design.)

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The PCI local bus is visible as one or more white connectors on a mother board, sitting parallel to the other ISA, MCA or EISA connectors that are on the board. These 32-bit connectors, or slots, are only about two-thirds the size of a 16-bit ISA connector.

They accommodate cards that may combine as many as four functions, such as disk controller, video and network adapter.

It is clear from the early PCI local bus system announcements that there are many different ways to build it into a system.

For instance, Advanced Logic Research of Irvine has introduced a new Pentium-based computer, the ALR Evolution V STP, which has four PCI slots and five ISA slots. They will probably charge a $300 premium for PCI over an otherwise comparable VESA VL design. But in prototype testing, the PCI design has faster video performance.

IBM Personal Computer has a PCI design, also using a Pentium chip, that incorporates two PCI slots plus three ISA slots. However, it also has built ATI’s PCI video chip into the mother board, connecting it to the PCI bus without actually using a slot to do so.

Clearly, there will be many variations in system designs. You won’t simply be able to count slots to determine how they compare.

In fact, when you count slots, here’s something to remember: In effect, there will always be one less slot than the total number of connectors. That’s because the PCI specification calls for one “shared” slot. This slot will have two connectors side by side, a PCI connector and a connector for the regular system bus, ISA or MCA or EISA. Either a regular expansion card or a PCI card can be placed in one of the two slots, but both cannot be filled.

For a while, we are likely to see manufacturers offering VL and PCI local bus systems, because the VL systems are cheaper to build and are comparable in mid-level machines.

But look for PCI local bus designs to dominate the high-performance end of the personal computing spectrum.