pcguide.com even mentions a 16MHz 486SX 🤣

They might be low power for notebooks or industrial / embedded computing applications.

I remember faintly from the early days that there where 486 machines with real 50MHz bus. However I believe stability was an issue. I remember that many of these 50 MHz machines had EISA mainboards?

I had the same problem, can't test my SX-16! But iirc I have read somewhere that the ASUS SP3 board has unofficial jumpers so you can set it lower. Mine blew up so can't test this myself. I need to go now and don't have a link but it was noted on winhistory.de, the pentium slowdown project 😉

Some (earlier/mid-time) 486 mainboards have a socketed crystal oscillator so you can define the clock speed by buying/salvaging the right oscillator. But you can try to overclock your CPU too.

Thanks a lot for everyone's replies. I decided to set the jumpers on a 486 mobo for a 25MHz CPU, and the (overclocked) 20MHz CPU POSTed OK. The CPU is OK. I may have an ASUS SP3 mobo in my collection. I can't remember. 🤣 If I find it, I'll see if I can figure out the 20MHz system jumpers! 😀

retro games 100 wrote:

Thanks a lot for everyone's replies. I decided to set the jumpers on a 486 mobo for a 25MHz CPU, and the (overclocked) 20MHz CPU POSTed OK. The CPU is OK. I may have an ASUS SP3 mobo in my collection. I can't remember. 🤣 If I find it, I'll see if I can figure out the 20MHz system jumpers! 😀

Back!

Heres the link that I mentioned earlier. Seems you can get the SP3 down to 8 Mhz!!
http://www.winhistory.de/more/386/xpmini.htm

It's in German so if your German isn't that great you might want to use google translate 😉

The Red Hill Guide suggested that most 486SX-25's were quite happy to run at 33MHz - indeed apparently a lot were illegally remarked and sold as a higher speed part. So I wouldn't be surprised at all if the 486SX-25's were higher speed parts that were binned by Intel to the lower spec. One could assume the same of the 486SX-20's.

Excellent - thanks a lot Tetrium! 😁

I know the SX-16 (which is the slowest 486 ever made) was only used in some Dell models. Usually normal motherboards won't even support it (officially). I'm not sure about the SX-20 though.
I do know that, because the SX-16 and SX-20 were only used by very few OEM's, those are also one of the rarest Intel socket 3 chips around. Not sure about the SX-25 but I reckon it's a lot more common. The SX-33 seems to be one of the most common 486 chips ever 🤣!

Anyway, if you ever get your hands on an SX-16 (or SX-20), you should definitely keep it! 😉

retro games 100 wrote:

Excellent - thanks a lot Tetrium! 😁

np mate! 😉
I should test my SP3 to see if it still works (after I remove the blown cap), it's a shame really. Imo it's one of THE best Socket 3 mobo's around.
(probably should delete my last comment 🤣, I feel that everytime we mention motherboard xxx is really great, Ebay prices for xxx will suddenly double hehe 😜 )

retro games 100 wrote:

Second question, please retro people! I am looking at a GA486IM Rev 2 mobo. It's a 486 mobo. The PCB states that a system clock speed of 50 MHz is available. Please can someone suggest some CPUs that would work OK at 50 MHz? I would then buy one of these CPUs, just to see it run at 50 MHz. It would be interesting. I could either overclock a CPU, or run one that is designed for 50 MHz system operation.

Thanks a lot. 😀

I know some Intel DX4-100 had the option to set the multiplier to 2x or 3x (for 2x50Mhz)
Ofcourse theres also the Intel DX-50 but that chip's quite uncommon (you might want to just overclock an AMD DX-40 if you have any vintage cpu love 😜 ).
Some steppings of the Intel DX4 can be set to 2x or 3x but not all. I'll try to locate a list somewhere, I know I wrote it down once.

Edit: Found the list, it was rather easy 😉
http://en.wikipedia.org/wiki/Intel_80486DX4

Edit2: Found some more info. Apparently the Cyrix 5x86 had a 2x multiplier as well, for 2x50Mhz.
Note: The AMD 5x86 does NOT have a 2x multi!
Links:
http://motherboards.mbarron.net/cputable.htm (also mentions some AMD DX4's have a 2x multi)
http://www.pcguide.com/ref/cpu/fam/g4C5x86-c.html

Awesome, thanks very much for the info! I was wondering, is it possible to do something a bit mad, and use either an Intel DX4-100 @ 50 MHz bus speed, or an AMD-X5-133ADZ (Am5x86-P75) @ 50 MHz bus speed? I am interested in seeing how far and fast a 486 can go! 😁

retro games 100 wrote:

Awesome, thanks very much for the info! I was wondering, is it possible to do something a bit mad, and use either an Intel DX4-100 @ 50 MHz bus speed, or an AMD-X5-133ADZ (Am5x86-P75) @ 50 MHz bus speed? I am interested in seeing how far and fast a 486 can go! 😁

You can overclock some AMD 5x86's to 3x50Mhz (some even to 4x40Mhz) but not all can be overclocked that much.

I did do some testing on a couple 486 mobo's of mine, trying out undocumented jumper settings to see what I could get (was trying to find 16Mhz fsb for my SX-16) and found out a couple of the most modern 486's actually have a setting for 60Mhz fsb! I can't remember really but one might have had a 66Mhz fsb). Anyhow, I only tested by booting, waiting till the speed was visible at the boot screen then shut down to prevent anything from burning up!
I posted it here around half a year ago?

Edit:Found the thread!
Link: Test:486 undocumented jumper settings
One board did actually have 60Mhz and 66Mhz jumper settings...according to the post screen at least

Edit2: Reading my old thread it would seem the maximum theoretical socket 3 cpu speed would be at 266Mhz! (4x66Mhz).

I had an AMD 486DX4 100 and I am 90% sure that I could select 2x or 3x multies because I remember trying it out as a DX2-50 and DX2-66.

So yea it would be interesting to see what scores a DX2-100 would get!