VOGONS


First post, by c0keb0ttle

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Ok, so i'm probably really thick, but I don't understand this.

I have this 486 motherboard:

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Here are the docs:
https://th99.bl4ckb0x.de/m/M-O/32216.htm

Now, it looks like this board should support all the way up to DX4 100 MHz CPUs. There are markings on the board for the settings, and also on the bl4ckb0x site.

However, when you look at the voltage jumpers it looks to me like they are fixed at 3.3V...

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But that's the settings for a DX4 100 MHz, right?

I have this board working fine with a DX 33 MHz.

Can I just put a DX4 100 MHz in there and it will work? Is it set for 3.3V or 5V?

The bl4ckb0x site says: "Note: The mainboard can only support 5v when a 3.3v regulator is not installed."

So does my board support 5v or not? Why is my DX 33 MHz working fine?

I'm feeling really thick and probably just missing something obvious. (Also, I was pretty sure I'd asked this question before but I couldn't find it so maybe I never did.)

Reply 1 of 10, by kanecvr

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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.

Last edited by kanecvr on 2017-10-24, 20:26. Edited 1 time in total.

Reply 2 of 10, by BitWrangler

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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.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 3 of 10, by c0keb0ttle

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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?

Reply 4 of 10, by ElectroMan

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‎‎‎

Last edited by ElectroMan on 2017-12-03, 13:31. Edited 1 time in total.

Reply 5 of 10, by BitWrangler

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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.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 7 of 10, by BastlerMike

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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.

Reply 8 of 10, by c0keb0ttle

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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?

Reply 9 of 10, by c0keb0ttle

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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?

Reply 10 of 10, by c0keb0ttle

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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.