Micronics 486 Motherboard Upgrade Results

Using 5 Volt and 3 Volt Upgrade CPU's

by Ben Myers

(C)Copyright 1996 Spirit of Performance, Inc.

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The table below summarizes the results of testing various Micronics 5 volt 80486 motherboards to date. It will be updated as required to reflect added information:

Board ID                   JX30G            JX30           Gemini         Baby Gemini                   
Part Number              09-00189-xx     09-00183-xx     09-00144-xx      09-00081-xx
BIOS Identification       JX30G-xx         JX30-xx          GLBxx         Gxx-2 RIDE

CPU
Cyrix Cx486dX2-66        MAYBE(Note 6)   MAYBE(Note 6)    MAYBE(Note 6)      Yes
Cyrix 5x86-100 (3V Note 2)  No              No              No (Note 7)      Yes
AMD Am5x86-133 (3V Note 2)  Yes             Yes             No (Note 7)      Yes
Pentium OverDrive (POD)     Yes             Yes             Yes              No (Note 5)

Notes:

All tests performed with Level 2 external cache enabled.
The Cyrix 5x86 and the AMD Am5x86 require a 3.3 to 5 volt voltage regulator to operate in 5v motherboards. Some voltage regulators may not allow a CPU to run at its full rated speed.
The Cyrix Cx486DX2 and 5x86-100 operate reliably in motherboards with either disabled external cache or no external cache. When the external cache is enabled, the motherboard often locks up prior to attempting to boot from floppy diskette.
The AMD Am5x86 is known in at least one instance to not operate with a BusLogic 445S VL-bus SCSI host adapter.
Baby Gemini boards have a fixed frequency oscillator soldered on at the factory, and operate at either 33MHz or 25MHz external CPU speed. They have a socket which accepts only CPUs with a 486 pinout. The baby Gemini motherboard used for testing has 64KB Level 2 external cache. (Does anyone have info regarding the three tag RAM chips required to set up the motherboard for 256KB L2 cache?)
The Cyrix Cx486DX2 boots and runs OK in some JX30G's and JX30's which we have tested, locks up per Note 2 in others. (Maybe this depends on the specific revision number of the board?)
There are two basic ZIF sockets used on the JX30. One has a plastic lever attached to both sides of the ZIF socket; the lever swings over the socket and does not provide enough clearance for some combinations of CPU and voltage regulator. (This ZIF socket is also used on the Gemini VL-bus motherboard.) The other has a single metal lever on the side and allows CPU with voltage regulator to be installed in the ZIF socket.
CPUs were not tested running overclocked, i.e. operating higher than the rated external clock speed. You can try overclocking CPUs, but you do so at the risk of having your system lock up unexpectedly. The following table shows DIP switch settings for selecting the external CPU bus speed on the JX30G motherboard. The settings for 40MHz and 50MHz do not appear in official Micronics documentation, but are silk-screened on the surface of some revisions of JX30G boards.

CPU Clock Frequency Selection

BABY GEMINI MOTHERBOARD -- (09-00081-xx)

A fixed frequency oscillator of either 25Mhz or 33 Mhz is soldered onto the board and is not switch selectable.

25Mhz MB's cannot be increased to 33Mhz without an oscillator change.

This MB contains a 168 pin PGA socket and ONLY supports 486 style upgrade processors and not the Pentium Overdrive CPU.

GEMINI MOTHERBOARD -- (09-00144-xx)

A fixed frequency oscillator of either 25Mhz or 33 Mhz is soldered onto the board and is not switch selectable.

25Mhz MB's cannot be increased to 33Mhz without an oscillator change.

This MB contains a ZIF socket and DOES support the 486 and Pentium Overdrive upgrade processors.

JX30 MOTHERBOARD -- (09-0183-xx)

DIP Switch 1

The following table shows DIP switch settings for selecting the external
CPU bus speed on the JX30 motherboard. The settings for 40MHz and 50mhz
do not appear in official Micronics documentation, and were derived through empirical tests.

External CPU Bus Speed   S1      S2     NOTES
---------------------------------------------
25MHz                    ON      ON     also DX2-50, DX4-75, 63mhz Pentium OverDrive
33MHz                    OFF     ON     also DX2-66, DX4-100, 83MHz Pentium OverDrive
40MHz                    ON      OFF    also DX2-80, DX4-120
50MHz                    OFF     OFF    (NO CPU found to operate reliably)

JX30G MOTHERBOARD -- (09-00189-xx)

DIP Switch 1

External CPU Bus Speed   S1      S2     NOTES
---------------------------------------------
25MHz                    OFF     ON     also DX2-50, DX4-75, 63mhz POD
33MHz                    ON      ON     also DX2-66, DX4-100, 83MHz POD
40MHz                    OFF     OFF    also DX2-80, DX4-120
50MHz                    ON      OFF    (NO CPU found to operate reliably)

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