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Asus 486SV1 will not detect 486DX50

M

MaTel

Experienced Member
Joined
Jun 16, 2008
Messages
183
Location
Hamburg, Germany
Hello,

I want to build a vintage EISA-System with a Asus 486SV1 VL/EISA Mainboard. In this system will only EISA-Components find their way in it ( NIC, GRAPHICS, SCSI ).

But I have some troubble with the Intel 486DX50 CPU. The Mainboard detect this CPU only with 33MHz. I can't find jumper settings to set the CPU-Clock to 50MHz.

I search for the Manual or anythink else to find the correct settings ( I know Hardware99, but there is no answer about my problem ).

Regards from Hamburg in Germany
 
TroyW said:
I'm guessing it's different to this one: http://total-hardware.info/m/A-B/31663.htm

I'll keep looking.

Oh, and not sure if you realised, but you'll have to find how to set it to run as a 25MHz, and it will internally clock double to 50 (it's 25MHz external, 50 internal, although some of them can be coaxed into working faster if you're lucky) :)
Click to expand...


Yes, this is the mainboard.
But my CPU need external 50MHz. Only the DX2-50 need 25MHz external.
In Total Hardware are many jumpers describt as: Factory configured - do not alter. I think some of them are for the external clock speed... but witch one?
In Total Hardware you can see, that the CPU jumper settings for DX16, DX20, DX25 and DX33 are the same. But how knows the mainboard the clock speed of the CPU? The 486 CPU has no CPU-ID.
 
MaTel said:
Yes, this is the mainboard.
But my CPU need external 50MHz. Only the DX2-50 need 25MHz external.
Click to expand...

You may still need to set it to 25MHz to get it to work as a 50Mhz CPU, that'll really depend on the mobo, obviously running it at 50 external would be preferable, but if the choice is between running at 33MHz total or 25/50, for most things the 25/50 would be much better.
 
TroyW said:
You may still need to set it to 25MHz to get it to work as a 50Mhz CPU, that'll really depend on the mobo, obviously running it at 50 external would be preferable, but if the choice is between running at 33MHz total or 25/50, for most things the 25/50 would be much better.
Click to expand...

OK. But I have no description how I can set the external clock on this mainboard. TotalHardware says nothing and on the mainboard is no printed info.
ALL DX16, DX20, DX25, DX33 CPU settings are the same.
 
Last edited:
Many pre 1993 motherboards required that the metal can oscillator to be changed in order to set the CPU clock frequency. According to TH99 your motherboard is one. Therefore you will need to change the 66MHz oscillator with a 50MHz one to get 50Mhz external clock and set the jumper accordingly.

If you have been attempting the 50MHz setting with the 66MHz crystal installed it will likely try to run the CPU at 66MHz (external); locking up the machine.

The jumper settings for CPU speed on your motherboard only determines how the oscillator frequency is used. All settings except for 50Mhz (external) divide the oscillator frequency in half. I suppose you could also use a 100MHz oscillator to get 50MHz as well (setting the jumper accordingly that is)
 
Anonymous Coward said:
Many pre 1993 motherboards required that the metal can oscillator to be changed in order to set the CPU clock frequency. According to TH99 your motherboard is one. Therefore you will need to change the 66MHz oscillator with a 50MHz one to get 50Mhz external clock and set the jumper accordingly.

If you have been attempting the 50MHz setting with the 66MHz crystal installed it will likely try to run the CPU at 66MHz (external); locking up the machine.

The jumper settings for CPU speed on your motherboard only determines how the oscillator frequency is used. All settings except for 50Mhz (external) divide the oscillator frequency in half. I suppose you could also use a 100MHz oscillator to get 50MHz as well (setting the jumper accordingly that is)
Click to expand...

OOhhh... OK.
There are two oscillators on the mainbord. OSC1 has 14,... MHZ and is soldered with the mainboard. OSC2 has 33,333 MHz and was inserted in a socket and secured with a cable strap.
I think I need a 50MHz oscillator to work fine with a DX50 CPU right?.
 
Last edited:
Thanks at all.

After I have changed the oscillator with a 50MHz version, the BIOS recognize the correct CPU Speed. The SI-Tool tells me also realistic values for my CPU.

regards
MaTel
 
Just make sure you tune the BIOS settings properly. You will need to make sure ISA is running between 10 to 8MHz by setting the divider. Also, you will need to use less aggressive SRAM and DRAM timings. If you stick with the default 33MHz timings you will probably have some stability problems.
 
Anonymous Coward said:
Just make sure you tune the BIOS settings properly. You will need to make sure ISA is running between 10 to 8MHz by setting the divider. Also, you will need to use less aggressive SRAM and DRAM timings. If you stick with the default 33MHz timings you will probably have some stability problems.
Click to expand...

the stability problems I have noticed. The System has crashed after it has tryed boot from disk.
Before changing the oscillator I have set the divider to 6. But the RAM-timings were to aggressive. After I have set it to automatic, the machine works fine.

Thanks!
 
Last edited:
You should try playing around and doing some benchmarking at various speeds. I found that on my EISA system with 50MHz external clock the memory performance was no better than a properly tuned 33MHz system because of all the extra wait states needed to get 50MHz working correctly, in which case you'd be far better off with a DX/2-66. For good 50MHz operation you'll probably need 12ns SRAMs. You can get them at JDRmicro, but they're not cheap.

If you really want to have some fun with your EISA board, try getting a VRM and using a DX4 in 50MHz clock doubled mode, or an am5x86-133 @ 150MHz in clock tripled mode. My own tests showed 4x40MHz to be the best performer. I found 40MHz bus to be the sweet spot. Fastest memory I/O, fastest stable speed for both system board and VLB. Despite what people say 50MHz VLB is quite a feat.
 
I have a 486DX50 CPU and all I want is this cpu work on my eisa mainboard. When I finished the "EISA-Projekt" the PC works as a backup-fileserver for my diskimages of my great vintage-software collection.
The operating system would be an "vintage" Novell-Netware 3.12 with a small boot scsi-disk and maybe 4 wide-scsi-disks mirrored ( 20gb each ) for the file server.
The data on the volume will be secured every month on a 24GB scsi-tape-drive.

It's a vintage file server for my vintage software collection :rolleyes:
 
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