Weird behavings SUNTAC 286 motherboard. \ VOGONS

Weird behavings SUNTAC 286 motherboard.

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First post, by Robin4

Posted on 2013-06-03, 00:57

Robin4 Offline

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Hi there,

Few months ago i bought a SUNTAC 286-12 motherboard from ebay..

Here is the sheet from this motherboard:

http://stason.org/TULARC/pc/motherboards/U/UN … ml#.UavUGT6poyw

Its for 90% the same motherboard as on that sheet here above in the link..

1. The first weird thing is, that this motherboard has a AMD 286 -10mhz processor in it.. I dont think this is standard on this boards, because i think they all where 12mhz versions i guess.. I dont had any opportunity to test the max speed of this board.. But my senses saying that its running 12mhz on a 10mhz processor.. Could it be that the previous user had to replace the processor (on a budget) So he just sticked a 10mhz one in and just let it running it on 12mhz speed.

2. I have some bios problems.. I have made some pictures, so its easier to explain whats its going wrong..
The problem is, when i start this board its counting all available memory.. At the first time it gave a bios error (had solder a new coin battery socket in, because it aint had any battery installed when i bought this board..
This board have a option for a coin type.. So i thought, better then this i shouldnt find.. (coin batterys have a very low fail rate,(i never havent seen a leaked coin battery, so the choice was easy to made)
But when it arrives to the bios it saying press `del` to enter bios.. But if i dont i really dont get any sponse (keyboard is in good condition).
I look likes the computer is hanging.. After a minute it gave me a second chance to get in the bios.. with F1 and that works..

Then it asks to choose from settings or bios diagnoses..

I can enter the settings, but cant get in the bios diagnoses.. Its saying that a wrong rom type [27128, 27256].. Ill ready checked if the rom type setting was set wrong.. But i tried both, but no progress at all..

I will upload the picture now..
The best things i got with this board..

Soldered coin battery works perfect.
Had some memory left (DIP 531000 ones) because the board doesnt support any sipps, and i had still some dip laying on me desk.. So i thought i just put them in this board.. Now its having a total of 4MB with Dips installed..

Tomorrow i wil upload the rest, now its a little late and need te sleep.

~ At least it can do black and white~

Reply 1 of 8, by h-a-l-9000

Posted on 2013-06-03, 16:57

h-a-l-9000 Offline

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Guess the diagnosis ROMs go into the two empty sockets. And are you sure the board doesn't charge your coin battery?
The long delay is most likely from the board looking for some drives that aren't attached.

1+1=10

Reply 2 of 8, by Robin4

Posted on 2013-06-03, 23:13

Robin4 Offline

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Thanks for reply.. No in the other emptys are used for 27128 eproms..

The emptys are for 27128 eproms
The full are for 27256 ones.. (thats are written on the PCB)

I exactly dont know if this motherboard charging the bios battery. Iam think its not.. Because the pcb have a circle on it.. So i guess its ment to use coin cell batterys on this board.. I havent had problems with it for now..So i leave it there till it gave me problems..

If iam looking the sheet it says that the wait times are 6 or 4.. So it shouldnt be a very fast motherbord to start up.

My idea what the best i could do with the bios problems..

- Maybe its a good idea to take them out and put them in my eprom writer. and just read them out.. I guess that the eproms could maybe fit in a 27128 one.(if the rom size its smaller then the actually rom chip size.)

~ At least it can do black and white~

Reply 3 of 8, by vmunix

Posted on 2013-06-05, 03:29

vmunix Offline

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as for the frequency check the crystals, there are two at least, one for the ISA bus and one for the CPU, the CPU could be rated at 20.000khz that means the processor is actually running at 10Mhz.

Trailing edge computing.

Reply 4 of 8, by Robin4

Posted on 2013-06-05, 09:45

Robin4 Offline

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one saying 24000 and a other 16000 (does are on the right side of the board)

The left one is a 14.31818

So i guess theres only one?

~ At least it can do black and white~

Reply 5 of 8, by Markk

Posted on 2013-06-05, 13:18

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I believe the 14.31818 is for the ISA bus and the other two for the CPU. One for standard speed (12MHz - 24000) and perhaps the other is for turbo off function(8MHz - 16000), or for a 287 math co-proccessor.

Reply 6 of 8, by vmunix

Posted on 2013-06-05, 15:00

vmunix Offline

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that's right, the 287 is asynchronous, so accordingly to the crystals you should be using a 12Mhz CPU and a 8Mhz 287.
If you have different speeds marked on the chips.. well you are overclocking a 286 🤣
try replacing the 286 with one @ 12Mhz or more, Harris CPUs can do 20Mhz or even 25Mhz for sure they can run at 12Mhz.

Trailing edge computing.

Reply 7 of 8, by Robin4

Posted on 2013-06-05, 22:18

Robin4 Offline

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Is it possible to replace the 287 crystal (i can solder)?

First i try to get that 10 mhz out and replace it with a 286-12.

~ At least it can do black and white~

Reply 8 of 8, by vmunix

Posted on 2013-06-05, 22:35

vmunix Offline

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of course, if you are lucky to have a 16 mhz 287 you can replace the crystal with a 32mhz, some motherboards had the crystal in a socket so it can be easily adjusted.

Trailing edge computing.

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