The board seems to be set to 3.3v by factory... it also looks like it has on board voltage regulator and n-gate transistors in the TO-220 form factor (above the lower ISA slot), so I'd bet it's set to 3.3v and does not support 5v cpus. As for why your DX-33 works... I have no clue, but it's happened to me before - plugged in a DX-33 on a mainboard that had an amd 586 in it and forgot to change the voltage to 5v - it worked fine. I only realized I didn't set the jumpers to 5v after I removed the DX to re-install the 586.

First time seing a n-gate mosfet on a 486 motherboard - usually it's just the voltage regulator, some resistors and maybe a simple diode.

Yah, later production DX33s could be run at 50, so they may have not minded undervolting either. Also it's possible that onboard regulator design is a bit basic and DX33 pulls little enough current that it doesn't regulate properly and is floating higher than 3.3. What that means is that you may not get the same experience with a DX2 66 5V.

Isn't it a bit strange though, that the board has all these markings which show how to setup SX, DX, and DX2, but then it's hard set for 3.3V?

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c0keb0ttle wrote:

Isn't it a bit strange though, that the board has all these markings which show how to setup SX, DX, and DX2, but then it's hard set for 3.3V?

Well by late 95, 96, all the CPUs in official "channels" were 3.3V, there was lots of surplus stock 5V ones booting around and getting sold at big discounts, but since official supply was 3.3, then locking motherboards for new builds to 3.3 wouldn't have been seen as a big deal.

So this is one of those "Socket 6" boards, eh?

BitWrangler wrote:

c0keb0ttle wrote:

Isn't it a bit strange though, that the board has all these markings which show how to setup SX, DX, and DX2, but then it's hard set for 3.3V?

Well by late 95, 96, all the CPUs in official "channels" were 3.3V, there was lots of surplus stock 5V ones booting around and getting sold at big discounts, but since official supply was 3.3, then locking motherboards for new builds to 3.3 wouldn't have been seen as a big deal.

We are talking about a mid-'94 board. Look at the dates on the chips.

Have you considered that this board may have an auto detection circuitry to select the proper voltage? Remember that 3,X Volt chips feature the "VOLDET" pin for this purpose.
You can see this on most of the later 486 boards by Asus which set the voltage by auto detection using a NDP406 as switching transistor.

Take a multimeter and measure the voltage while running a 5V chip and a 3V chip. Then you have proof.

BastlerMike wrote:

Take a multimeter and measure the voltage while running a 5V chip and a 3V chip. Then you have proof.

Could you give some tips on where on the board I can measure this?

Edit: I snooped around a bit and managed to find the info about shorting the VOLDET pin in the socket 3 and then measuring a Vcc pin.

Results are: Normal operation 5.46V, VOLDET shorted 4.29V.

Conclusion: Well, it detects something, but does it lower the voltage enough?

Bad form, replying to myself, but what the hey.

I downloaded the manual from here:
http://leoromo.net/mirrors/motherboards.mbarr … b3/ali486g.html

And it clearly states that if you have a 3.3V voltage regulator and diode installed at specific points on the board, it supports DX4-75/100. My board has those installed, and the fixed 3.3V voltage settings indicates it's an autodetecting board. I can confirm it works with both a DX 33 MHz, and a DX2 66 MHz.

I'm still not sure about testing it with the only functioning DX4-100 I have. There is a jumper (JK5) that you can set for "P24C" CPUs, or "Other" CPUs, and it is currently set to "Other". I assume I have to set that to "P24C" when using a DX4?

But I'm not sure about some of the other jumpers. What is an SL486? Should I set the jumpers for SL48DX/DX2/DX4 or something else?

Responding to my own thread again...

The board works great for DX4 100MHz. The CPU is "SL enhanced" according to http://www.cpu-world.com/sspec/SK/SK096.html so that gave my courage to use the SL486DX4 settings, and it works great!

So happy to have a board that can handle all sorts of 486 CPUs.