At 5V, it's probably not going to happen.
There were 3.3V DX/2s as well, which were basically rebadged or rejected DX/4s. I recommend use a VRM and run a DX/4-100 in 2x50 mode.

Thank you for the remark. I will check the voltage settings on the board if set to 3,3V or already to 5V.

I faced no problems running my Am486DX4-SV8B or Am486DX4-NV8T at 50x2 MHz.
However, you have to ensure that the board runs with 3.3 V and the multiplier is NOT set to x3.

And, yes, the cache and memory timings are a challenge.
On an EISA board I've noticed that soldering a dirty tag ram socket and populating it improves the performance at 50 MHz but not really at 33 MHz.
You may overclock ISA to 10 MHz. You should have board that can run PCI asynchronous at 33 MHz. You need to be very lucky if you get any VLB card running at 50 MHz.

Good luck!

Yes I will now ensure that I choose the right cpu type plus voltage and test it. Only using one vlc gpu trident tgui9440. Remember the Limitation could also come from my 486dx/2 66 cpu itself.

But now a bit confused. Should I overclock the CPU voltage to 5V or not? Seems quiteeeee a lot from 3,3v 😉

PS. In my award bios I can select various isa speeds: bus clock Divided by x/y/z or fix 7,14MHz (if I remember correctly)

Marco wrote on 2020-08-16, 08:41:

But now a bit confused. Should I overclock the CPU voltage to 5V or not? Seems quiteeeee a lot from 3,3v 😉

No one in this thread intended to talk about overvolting. Disruptor clearly said so in "you have to ensure that the board runs with 3.3 V".

The distinction between 3.3V and 5V was about the nominal voltage on the CPUs. The older 5V cores with bigger structure size were never intended to run at 100 MHz, and only do so if you are lucky. The new cores after the shrink are designed for both lower voltages (3.3V / 3.45V) and higher clock frequencies. So you will likely have better luck with CPUs designed for 3.3V than with CPUs designed for 5V operation.

You should definitely use an interposing socket that regulates the voltage to 3.3V when using the 3.3V CPUs. You can raise the voltage up to 3.6V without violating the specification of the 3.3V CPUs. Many boards provide a 4V setting, which most likely will not damage 3.3V CPUs in the short run (but it might speed up aging). Even if the 3.3V chips might work (for some time?) at 5V, I recommend that you use 5V only if you feel very brave and don't mind a fried CPU.

You mean overvolt. Do not do that. There are special motherboards that allow the voltages 3,6 and 4,0 Volt.
Always try to run the CPUs with matching voltage!

Please tell us the 5 digit number of the CPU. It is usually imprinted on the ceramics and should start with a 2.

ISA clocks:
25 MHz: stock clock is CLK/3 8,33 MHz, overclock would be CLK/2 12,5 MHz
33 MHz: stock clock is CLK/4 8,33 MHz, overclock would be CLK/3 11,1 MHz
40 MHz: stock clock is CLK/5 8,00 MHz, overclock would be CLK/4 10 MHz
50 MHz: stock clock is CLK/6 8,33 MHz, overclock would be CLK/5 10 MHz
If you do not have CLK/5 and CLK/6 you have to use 7,14 MHz

My EISA board requires to change the timings to 50 MHz before replacing the 33 MHz with the 50 MHz oscillator.

Very important: Do not trust what processor type and speed is printed on screen by your BIOS after POST.
I strongly recommend using benchmark programs like CTCM 1.7 (from heise) to verify clock and multipicator settings.

Sorry for my misunderstanding. Thanks so I will provide cpu and ctcm Data later and check again all correct jumpers.

My board supports cpu voltage selection 3.3/5V by >2 jumpers thus there might be hidden non documented settings behind. But that’s for Phase 2 maybe 😀

Besides NW I don’t have any speed relevant isa cards installed. Even onboard IDE is named VLB IDE port. Thus no need for overclocking here yet

Thanks again

Update. I found another manual with further voltage settings. Wow. But I will check if my board also supports this since I heard some chiconys don’t have the adequate VRM anymore equipped.

P1, P2, P3

1-2 for 5V CPU

2-3 for CPU as below

P4

1 o o 2 Close + 3,3 V

3 o o 4 Close + 3,45V

5 o o 6 Close + 3,6 V

7 o o 8 Close + 4,0 V

Note: Only one position can be closed in a time!

Marco wrote on 2020-08-16, 09:18:

Update. I found another manual with further voltage settings. Wow. But I will check if my board also supports this since I heard some chiconys don’t have the adequate VRM anymore equipped.

I found a photo of a board like yours in Want to mod 486 motherboard (Chicony CH-471B) to support 486 DX4 L1 WB. The VRM is the the 5-pin chip in the lower right corner, just behind the VL slot at the board edge. If that chip is missing, your copy of the board is 5V only. Furthermore, the board has 5 precision resistors (distinctive property: the blue case instead of the yellow/brown case) called R62-R65 as well as R69. The jumpers select one of R62/R63/R64/R65. The CPU voltage is determined by the ratio between R69 and the selected other resistor. If you have all five precision resistors on your board, your board does support four different "low-voltage CPU" voltages. As 3.3V/3.45V/3.6V/4.0V is by far the most common list of choices on boards offering four different voltages, I would blindly believe that your board also produces the voltages you quoted from the manual, and not a different set of voltages.

3.3V and 3.45V are both safe for "3V" 486-class CPUs, 3.6V is borderline and 4.0V is out-of-spec except for some variants of the Cyrix 486DX2/80.

Got it!!

Which CPU have you inserted now?

Sorry just saw that below the die there is nothing and the heartsink i won’t remove right now

CTCM reports CPUID: Cx486??/ Typ:00 fam:04 mod:00 step:00
FPU i487 type
L1 Cache 8KB 4-way WT

Version 1.5.

Update: system runs so far at 3,4V

Update:

System quite stable w/ 3,6V. But only with L2 disabled. A pity 15ns not being fast enough

Marco wrote on 2020-08-16, 15:40:

Sorry just saw that below the die there is nothing and the heartsink i won’t remove right now […]
Show full quote

Sorry just saw that below the die there is nothing and the heartsink i won’t remove right now

CTCM reports CPUID: Cx486??/ Typ:00 fam:04 mod:00 step:00
FPU i487 type
L1 Cache 8KB 4-way WT

Version 1.5.

Update: system runs so far at 3,4V

Update:

System quite stable w/ 3,6V. But only with L2 disabled. A pity 15ns not being fast enough

Chipset vendor recommendation for 50MHz FSB usually is a 3-2-2-2 burst (the slowest timing supported by most chipsets). Depending on your BIOS, it might just be called "cache timing: slowest", "L2 read timing: 3-2-2-2" combined with "L2 write: 1WS", "L2 timing: 3-2-3" or something like that. Just try to increase every cycle/wait state count that sounds related to L2 cache to the maximal value. You need to turn off auto-config to be able to manually configure cache and RAM timings (which you most likely should do anyway, as auto-config might guess that you are running at 33*3 instead of 50*2 and sets up FSB33 timings).

Yes fully agreed. I did all that. And at the end it works but very unstable w/ L2. Without seems ok.
Anyway the benchmark results were so close to the dx/2 80 with highest speed settings that I was really surprised. The synthetic mem and l2 benches of course even lower. So at the end no need to go for it.

Some motherboards perform better than others at 50MHz. I heard most 486 boards dont' support 50MHz properly, in that the cache and RAM will run so slow that 50MHz bus actually isn't faster than 33MHz. In my experience 40MHz usually provides the best memory performance on a 486. You'd be better to run a DX/4-120, or try to overclock a DX/4-100 to 120.

Marco wrote on 2020-08-16, 15:40:

Sorry just saw that below the die there is nothing and the heartsink i won’t remove right now […]
Show full quote

Sorry just saw that below the die there is nothing and the heartsink i won’t remove right now

CTCM reports CPUID: Cx486??/ Typ:00 fam:04 mod:00 step:00
FPU i487 type
L1 Cache 8KB 4-way WT

Version 1.5.

Update: system runs so far at 3,4V

Update:

System quite stable w/ 3,6V. But only with L2 disabled. A pity 15ns not being fast enough

As already stated - 50mhz is only turbo then you can drive all settings at max. otherwise go down a notch to 40mhz and work from there at full speed... hardware is way more likely to cooperate at a slower clock rate.
Maybe buy some srams(Cypress seem to be pretty good ones) and test - there are quite good 15ns pieces out there - you have to be patient.