6. System Board Jumpers

The system board sits on a REMOVABLE back-panel. (I didn't notice that). It makes it a lot easier to handle when setting jumpers. You know which way the board sits on the back-panel, from the keyboard socket and the case.

The details for your system board will very, but these are the sort of things you will see documented in the 'manual'. Go through the list of jumpers and connectors for your board.

Remember that the system board is "buried behind" the various ISA cards, so it's harder to access once it's installed. Make a note of the various locations of things on a photocopy of the board layout.

Also check the section about BIOS settings, and building up the system component by component.

6.1 ON-BOARD battery -vs- External battery.
6.4 486-DX4 clock multiplier Selector
6.5 CPU power
6.6 Cache Jumper
6.7 VESA clock
6.8 VESA wait state
6.9 Simms
6.10 System board connectors


ON-BOARD battery -vs- External battery.

The motherboard has a NON-RECHARGABLE battery, shipped as OFF.

You will have to remove the plastic insulator(s) and/or enable the jumper, so that the clock and CMOS work when the machine is switched off.



Tells the MB circuitry to drive one of:



Tell the MB circuitry to drive the CPU at (MHz).

486-DX-2 80 is 40 MHz


486-DX4 clock multiplier Selector

One of:

I presumed that this only applies when you have selected DX4.


CPU power

One of: (volts)


Cache Jumper

This is where you tell BIOS what kind of cache RAM chips are plugged in, and how much you have. EG 256 KB of { 64 128 256 512 1024 }

Cache memory is faster than SIMMS (20 ns < 70 ns), so repeated reads are taken from cache.

If you increase maim menory size above 16 MB, you should also increase cache memory.


VESA clock

I don't know what this does, but it says 40 MHz > 33 MHz -CLOSE-


VESA wait state

Adds 0 or 1 delay "wait state" in the middle of bus accesses.

wait states are an extra clock cycles that make bus accesses longer. This allows slower chips more time to read the BUS signals and respond to them. It also makes a difference in noisy environments, or at the other end of a long run, where the rising edge of a square wave, might not be clear enough, soon enough, to be reliable.

It is how a CPU running at 40 MHz (DX2-80), can use memory that is really only designed for 25/33 MHz.



You might not have to set any switches or jumpers, but will have to follow instructions in the system board manual, when purchasing and installing simms.

Pentiums require parallel banks of SIMM's, to get a 64-BIT path. My 486 is quite happy with a single SIMM and an empty expansion socket (or three). So you'll just have to jump straight to 32 MB.

There are also sub-types, and you may need to purchase the correct type. My (un-translated) M-Board says:

72 PIN SIMMS can be either "1 bank" or "2 bank" type.
If the 30 PIN SIMMS are also used, they will
be bank 2 of { 0 1 2 3 }, so that bank must be free,
ie of type "1 bank". This may be slower.

Ask the shop to explain.


System board connectors

Having set all the dip switches, and found all the connectors on the system board, install the back-panel into the chassis, connect up the speaker (if you want to), the two special power connectors from the PSU, the various disk LED's cables etc.

You will need a VGA card installed to see that the system board is OK.