How to upgrade your motherboard. (microcomputer motherboard) (Compute's Getting Stated With: Upgrading Your Processor)
by Mark Minasi

The rapid price drops in the PC world have put a lot of us in an uncomfortable position. What was a good buy just 24 months ago seems laughably lame now. PC prices have always dropped, but they have never plunged as they have in the past two years. As a result, many of us paid $2,500 for a fully loaded 386SX system in early 1991, and find it disturbing to open up a liquidator catalog and see that same system for $700.

It's disturbing, yes... but not unrepairable. If you bought a generic clone system (as I recommended in 1991 and recommend now), then the upgrade path is smooth and sure: get a new motherboard. Most clones use the same size motherboard, no matter which processor is on the motherboard.

For $350, you can move up to a 33-MHz 386DX system with the speed advantages that 64K of processor cache brings. For a steeper $900, you can get a 33-MHz 486DX motherboard with VESA local bus capability, offering the possibility of local bus video and blazingly fast Windows performance. Add another $200, and for $1100 you can get a motherboard that supports both VESA local bus and the advanced Extended Industry Standard Architecture (EISA) bus all on one board. A motherboard like that would be the foundation of one of the fastest PCs on the planet.

One problem with buying a new motherboard is that there are no big names in the motherboard business, no IBMs or Compaqs. Instead, there are several small and medium-sized vendors with products that are often good or excellent - but are sometimes quite bad.

Anyone can tell you that when you buy a motherboard, you must consider the processor chip, megahertz, and BIOS manufacturer: 486s are faster than 386s but are more expensive; more megahertz means greater speed but also greater cost. You'll also hear that you'll experience the least compatibility hassles if you stay with a BIOS builder such as Award, American Megatrends (AMI), or Phoenix. But there's more to buying a motherboard than BIOS compatibility. Here are some things to look for and things to avoid:

* Eight expansion slots. A number of motherboard manufacturers make their boards less expensive by omitting expansion bus slots. There's no point in forking over hard-earned cash for a mother board that won't let you plug in enough boards to take you through the next five years.

In just a few years, a baseline system will contain one board with the parallel/ serial/floppy/IDE/game-port interfaces, another board for video, perhaps another board for video capture, another for sound, a CD-ROM interface, and a LAN card (yes, even if you're in a small business or a home). That makes six slots the minimum. Eight slots guarantee that you can use whatever comes down the pike.

* Complete bus master support on EISA slots. You also should consider a motherboard that has EISA (Extended Industry Standard Architecture) compatibility, rather than the more common ISA (Industry Standard Architecture). It's only about $100-$200 more, and it assures that you'll be able to use the faster EISA boards as they become available. Many are already available, and their improved performance makes them worthwhile. But be sure that you get full-featured EISA slots.

One feature of EISA that makes it so fast is bus mastering, but not all EISA slots support it. In fact, I've seen motherboards that only support bus mastering on two slots - and these usually are the most easily accessible slots, which are needed to accommodate large boards such as ISA cards. Be sure that any EISA motherboard supports bus mastering on at least half of the slots.

* Watch for protruding connectors, jumpers, SIMMs, and DIP switches. Some motherboards are set up so that you can't install a full length card in two or three of the eight slots. When the SIMMs stick up an inch or two, it's impossible to use a full-length card that extends all the way down to the motherboard, as many do. Some motherboards have a cluster of Berg connectors (they look like metal pins sticking up off the boards; small black-plastic rectangular wiring connectors attach to them) that are for the reset switch, the turbo switch, the key lock, and the power light. Because the wires stick into this area, it's difficult to use full-length boards in this space. Other motherboards have their board clearance hampered by jumpers or DIP switches.

* The motherboard should be relatively inflexible. You actually can see light through some motherboards. These motherboards flex easily, and as a result, they warp when they warm up (any motherboard with a 33-MHz 486DX on it will warm up).

Along the same lines, be sure that there are enough support points in your motherboard case to accommodate at least seven of the little white nylon supports (these are the plastic things that hold the motherboard up off the metal case). There should be one for each corner, and three across the middle. You'll need these supports for EISA boards, which require additional force to insert, and local bus boards, which require a lot more force to insert. If you're pushing down on your motherboard and it does considerable flexing, you can have problems.

* The motherboard should be able to accept 256K of processor cache at least. Most motherboards on computers that run 33 MHz and faster need some high-speed memory called processor cache. It's a little bit of memory that's fast enough to keep up with the high speed of the processor. In general, the more cache memory you have, the faster the computer runs - up to a point. Beyond that point, the system doesn't get any faster, so you're spending money and getting nothing in return.

The point of maximum speed for most systems is about 64K if all you're doing is running DOS programs, or 256K if you're running Windows or OS/2. There are some inexpensive motherboards that only have room for 64K of cache; avoid them. Even if you don't buy the motherboard with 256K of cache RAM now, you'll want to expand eventually, and by then the prices may drop. Getting additional cache may not require spending that much more cash.

* If you buy a local bus motherboard, buy the local bus expansion boards from the same manufacturer. The whole idea with local bus is that instead of communicating with your system's cards at the mere 8 MHz that ISA and EISA run at, local bus boards communicate with the processor at up to 33 MHz.

Few people realize that local bus currently has a maximum speed of 33 MHz, unless the local bus board is soldered into the motherboard. That means that a 50-MHz system with local bus doesn't run the bus at the full 50 MHz. In fact, local buses typically must run at some factor of CPU speed - the local bus on a 50 MHz system would run at 25 MHz, 12.5 MHz, 6.25 MHz, or whatever. A local bus video card on a 50-MHz system actually would be slower than a local bus video card on a 33-MHz system.

Despite the fact that VESA has attempted to create a local bus standard, it's also standard to experience trouble trying to mix a local bus board from vendor A with a local bus slot on a motherboard made by vendor B. You're probably only getting local bus so you can get high-speed video anyway, so get the motherboard and the video card from the same vendor. And if possible, get a local bus video board built around the incredible ATI Mach32 chip.

Buying a motherboard can be rewarding. Not only do you get the benefits of a faster system, you also get the do-it-yourself satisfaction of having saved a few dollars on the upgrade. Happy upgrading!