Phil Storrs PC Hardware book

Notes on upgrading 486 PC Systems

486 processor based PC systems are now very old technology but they can often be put into service in areas that only require simple word processing and Internet access. In many cases they can be greatly improved by the addition of more memory and if the processor is a 486SX or 486DX33, a DX2 or DX4 chip will provide a worth while speed increase.

Changing the Processor chip

The following description is a guide to replacing an 80486 Processor with an Intel OverDrive chip or upgrading a 486SX or 486DX chip with a 486DX2/66 or similar.

Switch off the system, and unplug all the cables connected to it, both the power cable and the signal cables that go to the peripherals and make sure you note where each cable went. Slide the cover off the system's case. At this stage, and throughout the process, guard against dangerous static discharge by grounding yourself to the case chassis using a WRIST STRAP. The case should also be grounded to a good earth point. If your workbench is not equipped with a suitable earth point you could equip a three pin plug with only an earth wire and a clip on the other end to connect to the computer under repair.

Note where all the cables attached to the various expansion cards are attached, paying attention to the orientation of pin one for each cable. Disconnect cables attached to the Expansion Cards and then pull out all Expansion Cards. The Expansion Cards will have Hard and Floppy disk cables and Serial and Parallel cables.

Some PC cases allow easy access to the System Board with the board in place but if access is not that easy it is better to remove the board from the case. You may be able to gain better access if you remove the power supply. The System Board will have a speaker cable, one or more cables for the system lock, power led, turbo led, turbo switch and reset switch. If your system follows standard PC design, you'll find only one or two screws, usually one near the center of the back of the board and perhaps one near the center at the front of the board. Take out these screws and then either slide the board to the left to disengage the mounting posts or if the system uses the press fit mounting posts, squeeze each post with long nose pliers and press them out of the holes in the case.

Once the System Board is out, lay it flat on an Anti Static Bag on your work surface and connect your wrist strap to an earth pad near one of the mounting holes on the back of the board. The mounting hole near the keyboard socket usually provides an earth connection between the System Board and the computers case.

Chip Removal

If you are performing an upgrade, you must first remove it's original Processor chip. Make sure you have a suitable Chip Removable Tool. First, note that one corner of the CPU is bevelled to indicate its proper orientation. If the board is not marked by the dot on the Processor then make a mark with a Texta pen on the board. This mark will aid you in the installation of the new Processor chip. To simplify chip removal, Intel supplies a rake-like metal bar with each of its OverDrive chips. These tools are available from your Intel dealer. The teeth of the rake are designed to slip between the pins on the bottom of the microProcessor; this gives you better leverage when prying it out of its socket.

To remove your old Processor, hold the rake with its long arm angled upward, its teeth horizontal and pointing toward one side of the chip. Slide the teeth of the rake under the edge of the chip so they interleave with its pins. To do this, slide the rake parallel to the edge of the chip while pressing slightly inward until the teeth slip into place.

Now apply downward pressure to the long arm of the rake until you feel the chip give. Then lift the side of the chip, but only slightly. Trying to pry up one side all at once will bend its connecting pins; this will make the chip impossible to replace should you ever need to.

Move the rake to the opposite side of the Processor, and pry that side slightly upward. If the chip lifts easily, return the tool to the first side and pry a bit more. If the chip is reluctant, work your way around all four sides of the chip, prying each one upward in turn. In most cases, you'll need to pry each side about three times before the chip is loose enough for it to be lifted out of its socket.

If the computer has a ZIF socket, the process is far easier: Simply raise the lever at the side of the socket, and lift the old Processor free.

Once you've popped the Processor from its socket, hold the chip only by its edges. To store it, press the chip into a piece of black foam or slip it into one of the "chip carriers" Processor chips are supplied in.

Putting the new Processor in

Once the socket is empty, you're ready to slide the new Processor chip into place. Here, the only critical part is correct alignment.

Intel indicates the orientation of it's Processor chips in several ways. The most obvious is the bevel on the lower left corner of the square package. Turn the chip over, and you'll find that one of the pins has a square base and a small gold ray. These point toward the same corner of the chip.

Most CPU sockets have similar indications: a notched outside corner, a filled-in corner, or a legend or out-line silk-screened on the mother-board itself. Orient the Processor chip so that its notched corner follows the alignment indicated by the computers Processor socket.

Holding the Processor only by its edges, place the chip atop its socket in the proper orientation. With a ZIF socket, simply raise the lever; the chip should drop directly into place. Lock the lever, and your Processor is installed. With a conventional socket, you'll need to position the Processor carefully while inserting it. Make sure all the leads of the Processor match holes in the socket. Take care not to offset the chip by one or more rows in any direction.

When you're absolutely certain the Processor is properly aligned, begin to press it down into the socket. Start with firm, even pressure from your thumb or forefinger on the center of the chip. If it doesn't slide smoothly into the socket, most chips won't, work it down by pressing on each side individually, a small step at a time.

Ease the Processor down until the bottom of the chip nearly touches the top of the socket. Then inspect your work to ensure that none of the gold leads of the chip has been bent.

If any pins are bent, carefully remove the chip, and use smooth needle-nose pliers to straighten them out. Once you're sure the Processor chip is in place, put the computer back together.

Correct orientation

Make sure pin one on the Processor, is plugged into pin one on the socket. Pin one on the Processor is indicated by a small flat on the corner of the chip. The chip will look like a square with one corner cut-off - the cut-off corner indicates pin one. Watch out for the markings on the System Boards, some boards have the Processor marked as an X-Y matrix with numbers along one side and letters along the other side. Pin one does not usually correspond to the number one on the board. The Processor socket is usually marked with a square with one corner cut-off. Pin one of the socket is by the cut-off corner.

If you apply power to a System Board with the Processor plugged in the worng way around, the Processor or the System Board, or both, may be destroyed.

Changing the jumpers on the System board

. If you are replacing an Intel 486DX33 Processor with an Intel DX2/66 or OverDrive Processor you may not have to make any changes to the jumpers on the board. If you are replacing a DX Processor that runs at a different clock speed do not forget to change the jumpers that determine the Processor clock speed to 33Mhz.

Important - If you are replacing a DX40 or DX50 chip with a DX2/66 or DX4/100, make sure to set the clock speed back to 33MHz. You could if you are prepared for some failure try running these processors at 40 or 50MHz, a risky process known as over clocking.

Make sure the Processor has the correct operating voltage, remember some of the DX2 and DX4 chips require a lower operating voltage and the chip will be destroyed if powered up with 5 volt.

If you are replacing 486DX Processors from other manufacturers look for a different jumper configuration for these Processors than for an Intel device. Remember, boards without jumpers for the "other" 486 Processors may not work with these Processors.

You can see from this how important is to have the system documentation for the boards fitted to the computer. Many system boards will be of no use if you do not know the jumper setting for them and as this hardware is now very old, this information will not be available in most cases.

Summary of 80486 chips

The DX2 chips
The 80486DX2/50 and DX2/66 are clock doubling Processors that double the speed of the clock inside the Processor and operate at almost twice the speed of the corresponding DX Processor. The external clock speed of the DX2/50 Processor is 25MHz and 33MHz for the DX2/66. AMD and Cyrix have also produced DX2 Processors with AMD also producing a DX2/80 Processor. The Cyrix Processors are also sold under the IBM Blue Lightning name. Intels DX2 Processors operates from the usual 5 volt supply but some of the "clones" operate from either 3 or 3.45 volt and require a voltage regulator on the System Board, a feature not supported by any older System Boards.

Intel produced a range of DX2 Processors under the name OverDrive and these are basically 80486DX2/66 Processors with a built in voltage regulator and a heatsink. The idea behind the OverDrive Processor is they were a direct replacement for a 80486DX chip and provided almost twice the performance. Intel produced two versions of the 80486 OverDrive Processor to accommodate a range of Processor sockets found on early 80486 System Boards. Most 80486DX Processor boards will operate with an Intel DX2 or OverDrive Processor but this is not the case with the cloned DX2 Processors.

Intel, AMD or Cyrix Processors ?

If you look at modern 80486 Processor boards you will see jumpers that configure the board for various types of Processors including jumpers for Intel, AMD and Cyrix devices. This is because each Processor has it's own unique timing problems when it comes to accessing the memory and the I/O busses and this needs to be taken into account in the board design. As a general rule, if a System Board does not have jumpers for AMD and/or Cyrix Processors then it will not work reliably with these chips. The problems that occur with these Processors in such boards may not be immediately obvious and may only show up when the system warms up or with some combinations of instructions. This makes for a very frustrating time in coming to the conclusion about the success or otherwise of an upgrade.

Be careful of the System Boards that could take either 80386 or 80486 Processors. These are now quite old and many do not work reliably with even Intel OverDrive Processors. It is usually better to replace these boards with modern Local Bus System Boards, after all they have been in use for three or four years. Some of the early Upgradable System Boards even had a 80386 Processor surface mounted on the board and a series of jumpers to disable this "built in" Processor so the board could be upgraded.

The 486DX4/100 and DX4

The next development of the 80486 chip is a clock tripling chip called the 80486DX4/100. Intel uses a confusing array of names for this chip that include the P24C, 80486DX4/100 and DX4 OverDrive. Both Cyrix and AMD produce versions of this chip and each brand of DX4 has it's own particular requirements. Intels DX4 OverDrive has a built in voltage regulator and heatsink but most of the AMD and Cyrix chips require a regulator on the System Board and a heatsink and fan. Processors that operate from lower supply voltages have the operating voltage written on the top of the chip and can be either 3.0, 3.3 or 3.45 volt.

As a general rule, the DX4 chips only operate reliably in recent System Boards that may have been tested with these chips during the design stages and the AMD and Cyrix chips are more of a problem than the Intel chip when it comes to upgrading systems to DX4 specifications.

The Pentium OverDrive

The final development of the 80486 chip may be the Pentium OverDrive chip. Some System Boards have been produced that are equipped with a Pentium OverDrive socket. This chip took about two years to actually see the light of day and now the Pentium OverDrive chip has finally arrived it has been discovered not all the boards manufactured with a Pentium OverDrive socket will actually work with the chip. The Pentium OverDrive chip is a Pentium chip with a 32 bit external data bus instead of the Pentiums 64 bit data bus. See the Intel OverDrive Processor page for more details of these chips. For historical interest only.

Hard drive upgrade

Some years ago hard drive prices were almost proportional to the capacity of the drive but today the per Megbyte price falls rapidly as the drive gets larger. If we are replacing a hard drive in an older system today, we may come across older BIOS that do not have support for drives larger than 528 Megbyte. Older 80486 computers may not support LBA or similar technology. Look at the CMOS setup for the computer you are working on and see if it mentions Logical Block modes or large partitions. If it does not then you will have to use a utility like Disk Manager, available from the hard drive vendors, to provide support for large drives. This support is provided by modifying the boot record and partition sector on the disk and the procedure adopted depends on who's version of Disk Manager you are using. See the Enhanced IDE chapter for more detail on LBA and large partitions.

The battery and the Real Time Clock

You may not find a battery on modern System Boards, a plug in module from the Dallas corporation often supplies the RTC and backs up the CMOS setting. These devices have a battery life of in excess of ten years and should outlast most System Boards.

The Dallas module

Back to the upgrade chapter

Copyright © Phil. Storr, last updated 7th December 1998