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486 CPU questions

I

icono

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Oklahoma, USA
I am trying to upgrade a Packard Bell Executive 486SX 20Mhz with a faster CPU the included SX 20Mhz is just a bit below spec for some of the educational games my kids play on this machine.
I tried to buy a 486DX-33 and install it. It fit in the motherboard and works but only runs at 20Mhz(CheckIt was able to find the FPU).
the manual I found online says that the "CPU frequency cannot be changed" and to not use ODPs.

Do most people have success installing a DX2 in a motherboard that doesn't explicitly support it?
The DX2-66 seems to be more available I assume there would be no problems other than it would run at 40Mhz?
 
Correct. Just grab a dx2-50 instead. Usually they are a little cheaper. Other option be a UMC super 486, those tend to run faster then their rated speed vs Intel/AMD/Cyrix/TI/IBM.

Another option be using a couple wires to force the cpu into another front side bus speed, but not sure if your chipset will hold up or not.
 
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The clock signal should come either from a crystal oscillator or a clock generator. If you have sufficient soldering skills, it's probably possible to modify the board for a 25 or 33MHz bus.
 
Anonymous Coward said:
The clock signal should come either from a crystal oscillator or a clock generator. If you have sufficient soldering skills, it's probably possible to modify the board for a 25 or 33MHz bus.
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Given its stuck at one bus speed, and its age , you are dead on its mostly likely that it is a crystal and not even a clock generator. Question is does the chipset support any other dividers?

Op can you take a peak at the board or give us some good pictures of it? It help if we know what chipset it is to help you get the most out of the machine.








I
 
icono said:
The DX2-66 seems to be more available I assume there would be no problems other than it would run at 40Mhz?
Click to expand...
Yes, it would work this way. Note however that the DX/2 was the first to also exist in a 3.3V version. Since your mainboard supports 5V only, make sure you get the right one.

Also, a DX/4 Overdrive will work (ODPR, not ODP). This would give you 60 MHz (no matter if you use the -75 or -100 version).
 
Anonymous Coward said:
The clock signal should come either from a crystal oscillator or a clock generator. If you have sufficient soldering skills, it's probably possible to modify the board for a 25 or 33MHz bus.
Click to expand...
The tricky part for such motherboards with fixed bus speed is to make sure that the dividers of all components that rely on bus speed can be adjusted in BIOS. Or that they withstand the overclock of course, but then you enter overclocking territory where replacing a card can render the system unstable.
 
With a 5x86 upgrade (PowerLeap, Evergreen, etc.) it would quadruple the motherboard's clock speed, so you'd end up with an 80 MHz 5x86.
 
vwestlife said:
With a 5x86 upgrade (PowerLeap, Evergreen, etc.) it would quadruple the motherboard's clock speed, so you'd end up with an 80 MHz 5x86.
Click to expand...


I have an interposer for my 486 to throw in a 586. They were hard to find even when I got it years ago. Hate to see what they go for now. One I have is a Trinity works interposer, with a vrm.

This is the one I have, $150 bux now, eek! But its available... I'll try to dig out its manual tonight or find a copy to see what it supports. Also found out recently that the Dell I was using this on has a setting for POD that was undocumented, and supports 3.3/3.45 volts without it. So might not even need mine.

 
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Looking over the Motherboard I don't see any jumpers for bus speed only a set of three for "486SX/487/486DX".

The Crystal/Can is labeled 40Mhz
 
The 40MHz one is the one you need to replace. I'd desolder it, and put a socket in its place, that way you can revert back easily if need be.

If just stepping the bus speed up to 25MHz, it's unlikely that you'll be running anything very far out of spec. 8MHz is usually what ISA runs at, but there are at least three ways I am aware of that the ISA clock can be derived. The most common method is for the chipset to support ISA clock dividers, adjustable in the BIOS. For a 20MHz system, they would probably divide by 3 to get 6.67MHz (or maybe divide by 2 for 10MHz). Another method was to base the ISA clock on the OSC signal, which resulted in something like 7.2MHz. I've also seen the ISA clock run asynchronously, and it usually gets its own oscillator. In the last two cases the ISA clock remains the same regardless of the bus speed.
Let's assume that your system is configured to use chipset bus clock dividers. If it's set to /3, you'll be at 8.33MHz when running 25MHz system clock. For /2, you'd get a less desirable 12.5MHz, but there is a chance that it'll still work out okay. If you have an oscilloscope, you can measure the speed of the ISA clock before you mess with anything. There is also a guy online selling ISA cards that report the bus speed.
I know you have an OEM system, but if you go into the BIOS setup, do you see anything about ISA clock dividers?

Also, if you want to upgrade using an interposer, while relatively uncommon, I often see them pop up for under $40. Sometimes as low as $20. CPU-World is a good place to ask.
 
sorry for the late replay, the Packard Bell is a tightly packed OEM system these are the best photos of the Motherboard I can get. Without a more complete disassembly./
I checked every screen in the BIOS and I don't see any mention of bus speed/ divider
 

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Interesting early 486, the VLSI vl82c331 is a chipset normally seen on 386 systems, usually sx not 32 bit 486 machines (it doesnt do any memory addressing, and was usually dirt cheap)... And Intel memory controller, with Oak VGA. I've only seen that VLSI chipset on boards 16mhz on 386 systems (so 8 mhz ISA bus). Could be using it just as an ISA bus controller. Also see some rust between the Intel chipset and the memory slots you might want to take care of soon. Given all that, should clock to 25, but that OAK Vga might hold it back depending on the divider speed. To be honest if it supports a /3 divider should make a more stable system.

So that being said its up to you if you wish to replace the crystal oscillator. If you decide not to, go with an Intel branded 486 dx2, since they are 5v. And grab a better VGA card, that OAK is pretty terrible. Whatever you can find for a deal would be better, be it ATI or Tseng or even Trident.
 
It wouldn't suprise me the case had a lot of rust that I had to clean up. I'm not sure what I'm looking for in the photos. Are you able to circle it on the photo?
 
This computer also appears to have no L2 cache. Adding it will be hard since it uses one of those weird Intel cache modules.
Maybe an AMD 5x86 would be the way to go, since at least you'll have 16kb L1 to work with.
 
Those cache modules seem to be unobtanium, i have had an ebay search running for over a year and I haven't seen one yet( at a reasonable price)
 
I was able to find a DX2-50 Mhz and to works (at 40mhz) it's now fast enough that Check it 3.0 says the maximum performance has been reached and the lagging game work great.
 
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