left column : enabled, 250, 30, disabled, disabled, disabled, disabled, 0:300, enabled, On, disabled, C:-A:, High, enabled, enabled, enabled, setup, enabled
right column : disabled, disabled, disabled, disabled, disabled, enabled, enabled, enabled, disabled, enabled, enabled, 0 W/S, CLK2/1.5, 0W/S, 0W/S, W/BACK, enabled, enabled

what exactly are the options for bus clock rate select ? CLK2/1.5 ... ?

Thanks for the speedy reply weedeewee! And THANK YOU for the help!!

Options for the bus clock rate selections are as follows in order:
CLK2/1.5
CLK2/1
CLK2/2
CLK2/3
ClK2/2.5
CLK2/5
CLK2/6
CLK2/4

Ndc2012 wrote on 2021-03-13, 21:14:

Thanks for the speedy reply weedeewee! And THANK YOU for the help!! […]
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Thanks for the speedy reply weedeewee! And THANK YOU for the help!!

Options for the bus clock rate selections are as follows in order:
CLK2/1.5
CLK2/1
CLK2/2
CLK2/3
ClK2/2.5
CLK2/5
CLK2/6
CLK2/4

With the bus block rate selections, would you still select CLK2/1.5 as the best option?

If external cache is enabled but the system instable, try to add 1 waitstates at cache read cycle waitstates (and write as well). This can help if you have slow cache modules.
Also try CLK2/4 for 8mhz ISA Bus Speed. Everything above that might overclock anything ISA related which also can lead to instability.

pc-sound-legacy wrote on 2021-03-13, 21:34:

If external cache is enabled but the system instable, try to add 1 waitstates at cache read cycle waitstates (and write as well). This can help if you have slow cache modules.
Also try CLK2/4 for 8mhz ISA Bus Speed. Everything above that might overclock anything ISA related which also can lead to instability.

Thanks Pc Sound legacy!

Would I want it at CLK2/4 or 2/2? As far as I know the only 8-bit ISA card in there is the modem (which I obviously wont use), but the remaining ISAs are 16-bit (The VLBs being 32). I'm sure I'm the wrong one here, I just wanted to make sure!

Thanks!

And I would disable memory parity check if you have "normal" ram installed. Parity check is kind of ECC option ram that is not very common.

pc-sound-legacy wrote on 2021-03-13, 21:39:

And I would disable memory parity check if you have "normal" ram installed. Parity check is kind of ECC option ram that is not very common.

Good question, I am not completely sure about it. But to much bus speed may cause the hdd controller trouble so I would start with a low ratio and then optimize it until one step before instability starts. A higher divider leads to lower mhz and that won't be a problem but slow. A divider too small leads to overclock es bus speed and some controllers/cards don't like that.

pc-sound-legacy wrote on 2021-03-13, 21:43:

pc-sound-legacy wrote on 2021-03-13, 21:39:

And I would disable memory parity check if you have "normal" ram installed. Parity check is kind of ECC option ram that is not very common.

Good question, I am not completely sure about it. But to much bus speed may cause the hdd controller trouble so I would start with a low ratio and then optimize it until one step before instability starts. A higher divider leads to lower mhz and that won't be a problem but slow. A divider too small leads to overclock es bus speed and some controllers/cards don't like that.

THANK YOU, seriously. I've been having a hard time understanding some of the maths with all the different bus speeds at play on these old machines. This helps immensely. I'll start playing around, and I'll report back!

Well, Ndc2012, we don't know what CLK2 is so.... if it's 33MHz (the cpu clock), then best to set it to CLK2/4 just like pc-sound legacy suggested already.
Anything over 8MHz is already overclocking the poor ISA bus.

Thank you wedeewee & pc-sound-legacy!

Still encountering some issues, so I'm not entirely sure where the problem lies. I see now, and definitely agree that the bus clock rate select should be CLCK2/4. Weedeewee, I input the values you suggested, and modified the bus clock rate to clck2/4, but it seems when I enable Burst Write and CPU write back cache, the system crashes and resets the BIOS completely down to the date. If I disable them, but keep the rest of the settings as suggested, the BIOS retains everything, but hangs when trying to install anything - even DOS. I've also tried adding 1 wait state to both the Read and Write cycle of the cache, but that doesn't seem to do anything either. At any point should I consider the chips themselves to be bad?

Also: I didnt mention earlier, but the system also has a Turbo button. I'm not sure if that's interfering with any of the configurations either, but I don't see anywhere in the BIOS where I would configure that.

Just a guess, are the jumpers for cpu selection correctly set ?

weedeewee wrote on 2021-03-14, 01:01:

Just a guess, are the jumpers for cpu selection correctly set ?

From what I can tell, yes. The jumpers are set for a 486DX2. I'm going to include a pic of the board itself. It's the whole board, so some zooming might be required, but jumpers JC1-JC5 are set for a 486DX, and bank RN9&10 were left empty to indicate a 486DX2.

Sorry for the quality of the pic in advanced, but it's the only clear one I have, aside from dismantling the system again.

Ndc2012 wrote on 2021-03-14, 01:13:

weedeewee wrote on 2021-03-14, 01:01:

Just a guess, are the jumpers for cpu selection correctly set ?

From what I can tell, yes. The jumpers are set for a 486DX2. I'm going to include a pic of the board itself. It's the whole board, so some zooming might be required, but jumpers JC1-JC5 are set for a 486DX, and bank RN9&10 were left empty to indicate a 486DX2.

Sorry for the quality of the pic in advanced, but it's the only clear one I have, aside from dismantling the system again.

Added for clarity (since the picture angle sucks): I've noticed that on Jumper JCx1 that there is a jumper covering pin 1, but not pin 2. I don't actually see this as a configuration in the documentation I've found online. According to the documentation, if pins 1&2 at JCX1 is open it indicates an Intel S-series CPU, if it's closed it indicates a CX486S. But it doesnt say anything about what covering pin 1 and not pin 2 does. At JX1 pins 1&2 are closed to indicate a 1x CPU clock mode, and JV1&2 have pins 2&3 closed to indicate 0 VESA wait states for <33MHz CPU speed.

As far as I know, the only jumper I've modified on the board is the one that sets the CMOS battery selection from on-board to external.

Ndc2012 wrote on 2021-03-13, 20:42:

Hi Everyone! […]
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Hi Everyone!

Long time reader, first time poster - I'm hoping y'all can help (sorry for the lengthy post in advanced)!

I have an old 486 clone machine that my family got for Christmas in 1994. I just found it in a closet and have been working to rehab it. I didn't start building machines until the Athlon era, so my knowledge on the 486's is a bit more limited, but I truly cherish the tech.

Here are the specs of the machine:
486-GVT (Rev A2) Motherboard (Not the more popular 486-GVT2 board, I'm basing my findings on the TAG placement for the external cache configurations)
AMD 486 DX2-66
16MB FPM RAM (4 x 4MB 30 pin with parity chips 4x9 @60ns)
256KB External Cache (8 x AS7C256-20PC chips + 1 for TAG) Alliance Chips
LSI Chicago 235 watt PSU
The board has an AMIBIOS chip reading 486DX ISA BIOS AB5649161

Expansion Cards:
Soundblaster 16 CT2770 ISA sound card
Cirrus Logic CLVGA542XVL/H VLB Card with 1MB onboard
QD6580 HDD + Floppy VLB Controller card
UMC PC1414UX 14.4 ISA Fax Modem Card

Updates:
I've replaced the mechanical HDD with a Compact Flash storage device (Any drives above 500MB require a Disk Overlay, unfortunately the system doesn't support LBA natively)
I've ordered a 64MB FPM Ram kit from ebay, but it hasn't arrived yet
I removed the double speed CD rom drive (which still worked), and replaced it with a 24x drive from the early 2000s

When I found it, the CMOS battery was dead as a doornail, and thus the BIOS had lost all configurations. I've since removed the soldered battery, and replaced it with an external one, but I am having a HECK of a time with the bios configuration.

When I use the "Power On Defaults" the computer functions, albeit at a glacial pace. Power on defaults disable both the internal and external cache memory. I've tried using "BIOS Defaults" setting, which does enable both internal and external cache, but the system is highly unstable, and cannot complete any windows installations. I found that I can use the "Power On Defaults" and enable the internal cache, and the computer will continue to function, but as soon as I start enabling the external cache is where I start running into system crashes and instability. So I'm sure it has to do with the way the cache is (not) being configured in the BIOS. I will attach photos of the Advanced Configurations for "Power On Defaults" and "BIOS Defaults" screens, plus the POST screen for the BIOS version details.

Can anyone help me out with the configurations here to enable both the internal and external cache so the machine doesn't hang or crash? I've done hours of research online, but I'm still not sure that I understand the math for the wait states with all the different bus speeds and the CPU clock multiplier, or whether I should be using write-back or write-through schemes.

Any and all help would be GREATLY appreciated. Thank you for all that you all do, I honestly wouldn't have been able to make it this far without this forum!! Please also let me know if you need any further details, I tried to provide everything I felt would be necessary for diagnosis, but can also do my best to provide whatever else you might need!

Ok, after further review and playing around with settings, I'm starting to wonder if one (or several) of the external cache chips is bad? I incorporated almost all of the settings provided in the threads, except I disabled external cache and CPU write back cache. When I do that, the computer functions quite beautifully, it installs Windows 3.11 and Windows 95 in 30 minutes flat with no system instability. Running most programs is a breeze, but there are some period games that are having conventional memory issues. As soon as I reenable external cache, the system begins to hang, crash and reboot. I've tried adding wait states to no avail, I tried changing the external cache scheme from write back to write through but that just causes the system to hang at POST and not even make it to Windows. Could it be possible that one or several of the external cache chips is bad? I dont know the probabilities of that happening, so I'm not sure.

Thanks all!

I found your board on Ultimate Hardware here http://www.win3x.org/uh19/motherboard/show/2382

Would you like to dump your bios and share it with us ? If you don't know how to do it I'll guide you through.

Deksor wrote on 2021-03-21, 00:40:

I found your board on Ultimate Hardware here http://www.win3x.org/uh19/motherboard/show/2382

Would you like to dump your bios and share it with us ? If you don't know how to do it I'll guide you through.

If you wouldn't mind walking me through, I'd be happy to oblige, Deksor!

Thanks for the reply, and offer to help!

Allright !
So download this archive http://cd.textfiles.com/microhaus/mhblackbox3 … MORY/GETROM.ZIP
Put it on a bootable DOS disk and run ROMSAVAT.EXE. It should generate an image of your BIOS. Then save it to your modern PC and send it here 😀