It might be that the table does not really specifies the FSB clock, but instead tells the CPU internal clock speed.
So the DX2-66 would need 33 MHz clock really, but you look in the table for 66MHz settings from the top half of the table.

If you had 40MHz CPU it might be either clock-doubling internally, or running directly at 40MHz. Hence the two ways to "set 40MHz CPU" - one to give the frequency as is (the bottom half of the table) or give 1/2 of the rated frequency for clock-doubling CPUs (top half of the table).

Thanks for the reply. That indeed sounds plausible. I was wondering before, whether this was the case. But then I thought, "who would write such a poor documentation?". How were system builders back in the day supposed to know, which is which?

Incidentally, when i look up the spec sheet on th99, it even says 50iMHz, which I now interpret as "internal only".

I guess if I had an actual 486DX-50 I could compare both settings via benchmark. Considering that 50iMHz would mean that FSB is running at 25 MHz I should see quite a performance loss vs the other setting.

maxxis486 wrote on 2023-03-25, 19:52:

Thanks for the reply. That indeed sounds plausible. I was wondering before, whether this was the case. But then I thought, "who would write such a poor documentation?". How were system builders back in the day supposed to know, which is which?

Incidentally, when i look up the spec sheet on th99, it even says 50iMHz, which I now interpret as "internal only".

I guess if I had an actual 486DX-50 I could compare both settings via benchmark. Considering that 50iMHz would mean that FSB is running at 25 MHz I should see quite a performance loss vs the other setting.

The chart indeed has way too many choices. I wonder if it was copied from the datasheet of the clock generator chip rather than being rationally created for different 486 chips.

for a pre-DX4 board like that there should only be a few choices: 486SX/DX-25 or 486DX2-50 (25 MHz bus), 486SX/DX-33 or 486DX2-66 (33 MHz bus), 486SX/DX-40 or 486DX2-80 (40 MHz bus), and 486SX/DX-50 (50 MHz bus). Not 11.

Yes, a 486DX-50 should be faster than a 486DX2-50. The problem is that a 50 MHz bus requires de-tuning the cache and memory burst settings. The 486DX2-66, which cuts the bus back to 33 MHz and allowing more aggressive timings, is more of a happy medium.

All of the 40 MHz and 80 MHz stuff was from AMD and Cyrix and not Intel.

Another explanation could be that perhaps they copied portions of the manual from a previous offering of those 386/486 combo motherboards. That would explain needing more choices as a 386 chips is "clock-halved" in that the external clock is twice the frequency of the CPU.

jakethompson1 wrote on 2023-03-25, 21:19:

for a pre-DX4 board like that there should only be a few choices: 486SX/DX-25 or 486DX2-50 (25 MHz bus), 486SX/DX-33 or 486DX2-66 (33 MHz bus), 486SX/DX-40 or 486DX2-80 (40 MHz bus), and 486SX/DX-50 (50 MHz bus). Not 11.

Yeah, that's actually what I would've expected.

Yes, a 486DX-50 should be faster than a 486DX2-50. The problem is that a 50 MHz bus requires de-tuning the cache and memory burst settings. The 486DX2-66, which cuts the bus back to 33 MHz and allowing more aggressive timings, is more of a happy medium.

Yes, I read about that on here somewhere. Makes total sense when you think about it.

All of the 40 MHz and 80 MHz stuff was from AMD and Cyrix and not Intel.

Yeah, those were the ones I grew up with 😀 I still have fond memories of my AMD 386DX-40, which I didn't replace until the AMD 486DX2-66 came around. Those were the good old times 😉 After that there was Cyrix M-II and AMD K6, but was never quite happy with those. For me, things started to look good again when Pentium II/III on BX440 became affordable. Kept both, the 486DX2-66 and P3 rig up to this day 😀

Please excuse my rambling 😀

So, today I finally had a chance to do some testing with different CPUs (486DX-33, 486DX-50, 486DX2-50, 486DX2-80)

I tried all 16 possible jumper settings using the following CPUs and here's what I found out:

The clock speed settings, as documented in the manual, all refer to the base clock speed (CLK2), which is derived from a 14.383 MHz crystal .
That means, that setting, for instance, the clock speed to 40 MHz would run a 486DX at 40 MHz, while running a 486DX2 at 80 MHz.
As you can see in the piture below, this was the case for all possible base settings.
And yes, that would mean that the board also produces 66 Mhz and 80 MHz base clocks even though, as far as I know, there never was a corresponding 486DX or DX2 supporting those base clocks.
Naturally, when I tried setting the base clock to 66 MHz or even 80 MHz, the system wouldn't POST.

Last but not least, I also found some undocumented clock speeds - namely 12 MHz, 16 MHz, 20 MHz, 24 MHz and 32 MHz.
With a DX installed, only 24 MHz and 32 MHz would actually work. The BIOS would still say 25 MHz DX, but SysInfo correctly showed 24 Mhz, which corresponds to the benchmark score.
With a DX2 installed, all base clock speed where possbile, resulting in 24 MHz, 32 MHz, 40 MHz, 48 MHz and 64 MHz respectively.
I assume, that those lower base clocks where too low even for the DX-33, since the system would POST but then hang when trying to boot from floppy.

Edit: After disabling auto configuration in the BIOS I could get the system working with the lower (undocumented) clock speeds.

Here's an updated overview of the settings and witnessed outomes:

Hope this is of interest to someone out there 😉

Regards,
Rene

Well, your manual really looks strange. In its table it talks about CLK2.
Well, in 1993 there have been clock generators widely available for use with 386 CPUs. They have needed CLK2 input which is double frequency.
I'm not sure which 486 processors used CLK2 input, but the most common 486 do use CLK input, single frequency.
I just think KMD put an available clock generator on your board.

So don't bother too much and don't damage your board 😉

However, at least your 486DX 33 is specified from 8 to 33 MHz. If your board refuses at those low clock frequencies, it's not a problem of the CPU.
Do you have a port 80 diagnostics card?

Yes, I actually do have a diagnostics card here. So far I didn't have to use it very often which is why I totally forgot about it.

I will give it a try with some of those low frequencies. I am really curious though, as to why they would provide base frequencies of 66 and 80 mhz back then. Was it to be future proof? At least the 66 mhz could be needed for the Pentium overdrive, I think.

As for the clock generation: I've had people mention that on the earlier boards there would be a crystal with 2x the actual CPU clock, from which all other clock speeds were derived. This meant that in order to upgrade your CPU back then you had to swap out the crystal. I assume that they later went with the more general purpose crystal of 14.383 and use internal logic to derive all kinds of clock speeds using division and multiplication. I just didn't expect it on such an early board (the Bios is from 1992).

Also, they kept the wording CLK2 in the manual as well as in the BIOS settings, when setting the clock ratio for the internal bus, which I assume is the ISA bus, since the VLB should run with full clock speed.

Yes, the crystals have been until one generation before.

maxxis486 wrote on 2023-04-13, 06:12:

Yes, I actually do have a diagnostics card here. So far I didn't have to use it very often which is why I totally forgot about i […]
Show full quote

Yes, I actually do have a diagnostics card here. So far I didn't have to use it very often which is why I totally forgot about it.

I will give it a try with some of those low frequencies. I am really curious though, as to why they would provide base frequencies of 66 and 80 mhz back then. Was it to be future proof? At least the 66 mhz could be needed for the Pentium overdrive, I think.

As for the clock generation: I've had people mention that on the earlier boards there would be a crystal with 2x the actual CPU clock, from which all other clock speeds were derived. This meant that in order to upgrade your CPU back then you had to swap out the crystal. I assume that they later went with the more general purpose crystal of 14.383 and use internal logic to derive all kinds of clock speeds using division and multiplication. I just didn't expect it on such an early board (the Bios is from 1992).

Also, they kept the wording CLK2 in the manual as well as in the BIOS settings, when setting the clock ratio for the internal bus, which I assume is the ISA bus, since the VLB should run with full clock speed.

Hello,
That is really interesting.
I have NEVER seen a 486 board with an official documented setting for external 66MHz. And here there is even an external 80MHz setting??? That is really super weird.

EDIT:
So with the 80MHz and an Am5x86 CPU at multiplier 4, it would be 320MHz. Very cool 486 machine that would be.

CoffeeOne wrote on 2023-04-13, 19:53:

Hello, That is really interesting. I have NEVER seen a 486 board with an official documented setting for external 66MHz. And her […]
Show full quote

Hello,
That is really interesting.
I have NEVER seen a 486 board with an official documented setting for external 66MHz. And here there is even an external 80MHz setting??? That is really super weird.

EDIT:
So with the 80MHz and an Am5x86 CPU at multiplier 4, it would be 320MHz. Very cool 486 machine that would be.

Yeah, that is weird. Too bad I don't have an oscilloscope so that I could measure the actual frequency on the cpu clock pin.

Still trying to get it to run at the undocumented frequencies below 20mhz just out of curiosity. However, the POST stops at code A4, which sould be setting up wait states. Will play around with BIOS some more.

Fun fact: the machine does enter the BIOS settings without trouble and works great while fiddling around in the BIOS settings. It's only when it is supposed to start booting after Floppy und HDD have been properly initialized, that it stops..

maxxis486 wrote on 2023-04-14, 12:54:

Fun fact: the machine does enter the BIOS settings without trouble and works great while fiddling around in the BIOS settings. It's only when it is supposed to start booting after Floppy und HDD have been properly initialized, that it stops..

Oh, then you could try to disable the caches, play with waitstates, ...
Do you use auto config?

I guess there are no auto config table entries in your BIOS for that slow frequencies...

Disruptor wrote on 2023-04-14, 13:31:

Oh, then you could try to disable the caches, play with waitstates, ...
Do you use auto config?

I guess there are no auto config table entries in your BIOS for that slow frequencies...

As a matter of fact, I do use auto config, though I am not sure what exactly is being automatically configured here, since I also set the clk ratio and ws manually. Or may be those settings don't do anything when auto config is enabled.. I will try disabling auto config.

Thanks for that tip! With Auto config option disabled the system boots without any problems. All undocumented clock speeds (12, 16, 20, 24 and 32 MHz) work as expected. I wonder, which options are being automatically configured, since none of the individual BIOS options are disabled when enabling auto config.

Here's a screenshot of my current settings when running on 12 Mhz.

I think the ISA bus could be a little underclocked at 12 Mhz, since you can even see the lines being drawn from left to right 😁

So basically, the documentation in the original manual is not only confusing (having multiple numper settings for the same clock speeds without any explanation), it's also incomplete, as I know now.

Fun fact: with this board an a DX-50 I can basically emulate any old system from 12 Mhz to 50 Mhz without having to change board or CPU 😀

Thanks again for you help!

I guess all that's left is to start playing some games and see how those feel with the different clock speeds 😀

Cheers!

maxxis486 wrote on 2023-04-14, 14:42:

Fun fact: with this board an a DX-50 I can basically emulate any old system from 12 Mhz to 50 Mhz without having to change board or CPU 😀

According to the datasheet: No 😀
The Intel 486DX-33 is specified for 8 - 33 MHz.
The Intel 486DX-50 is specified for 16 - 50 MHz.

But you can try to disable cache(s). That easily will turn your 486 into a 386sx.

Disruptor wrote on 2023-04-14, 18:10:

According to the datasheet: No :) The Intel 486DX-33 is specified for 8 - 33 MHz. The Intel 486DX-50 is specified for 16 - 50 MH […]
Show full quote

maxxis486 wrote on 2023-04-14, 14:42:

Fun fact: with this board an a DX-50 I can basically emulate any old system from 12 Mhz to 50 Mhz without having to change board or CPU 😀

According to the datasheet: No 😀
The Intel 486DX-33 is specified for 8 - 33 MHz.
The Intel 486DX-50 is specified for 16 - 50 MHz.

You're right. I initially tried those low clock speeds with my 486DX-33.

So I went back and installed the 486DX-50 and yeah, it also works with 12 MHz 😀

It sure is very slow though... takes forever to check the 16megs of RAM 😀

So, I sure be using this setting, but it's nice to know that it does actually work.

Disruptor wrote on 2023-04-14, 18:10:

But you can try to disable cache(s). That easily will turn your 486 into a 386sx.

Yeah, that I know.. but I find that by adjusting the clock speed I have more direct / finer control over the overall system speed.

As you can see, the SysInfo benchmark scores scale up quite linear:

12 MHz 25,9
16 MHz 34,5
20 MHz 43,2
25 MHz 54,0
33 MHz 72,0
40 MHz 86,0
50 MHz 108,0

In fact, the most interesting part to me is that this board, having a 486 socket, actually allows changing the speed without having to swap out any crystals.

But then again, this could have been common practice for 486 boards back then, I haven't checked that.

Edit: the CPU is an actual i486DX-50

Needless to say it barely gets warm when running at 12 MHz 😀

maxxis486 wrote on 2023-04-14, 23:32:

So I went back and installed the 486DX-50 and yeah, it also works with 12 MHz 😀

Fine.

maxxis486 wrote on 2023-04-14, 23:32:

Needless to say it barely gets warm when running at 12 MHz 😀

A rule for the 5 Volt 486 is:
<= 33 MHz: no Heatsink
DX-40, DX2-50: Heatsink
DX-50, DX2-66: Heatsink+Fan

Disruptor wrote on 2023-04-15, 04:12:

A rule for the 5 Volt 486 is: <= 33 MHz: no Heatsink DX-40, DX2-50: Heatsink DX-50, DX2-66: Heatsink+Fan […]
Show full quote

A rule for the 5 Volt 486 is:
<= 33 MHz: no Heatsink
DX-40, DX2-50: Heatsink
DX-50, DX2-66: Heatsink+Fan

Thanks, that's good to know.