In some of my boards the two grouped pins are ground and the the lonely one is +.
Check with a multimeter.
Maybe diodes need to be added. IDK

For 6V I don't know.

Rolling,

Dont use 4x1.5V AA batteries, use only 3.

when they are new they have 1.6V each.Even with 6 V, it is already too much voltage.

Can you take photos on top and bottom sides of the P1 area ?

The "key" (missing) pin doesn't have anything to do with the keyboard, it's there so that you don't mix up VDC and GND. The idea is that the connector wouldn't allow you to plug it the other way around, now of course you don't have the actual pins and the battery connector for this to work.

Most external battery connectors were meant for non-rechargeable batteries, but for machines of that age you can't be certain. Isn't it fairly simple though to measure if charging current reaches these pins?

I checked it - Pin 3 & 4 are GND and Pin 1 is +, as mentioned in the manual and by Nexxen. There seems to be no charging current.

I'm still not sure about the voltage though. I've had 286 systems which needed 6V to hold CMOS settings, but all of them had some diodes beside the ext. battery pin header.

Since the spots for diodes on this board (D3-D6) aren't populated and even one joint at D6 is bridged with wire i'm not sure, if i can just put 6V there.

Yes there are 1 or 2 diodes in series, but it's better to play it safe...

So, use 3 batteries and you won't have any problems.

You have only to pay attention if any charging circuit is present.

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Do any of you think that adding an external battery by adding an unpopulated external battery connector to a mainboard that uses a "DS1287 RTC with integrated battery and no external battery pins" will have the effect you desire of keeping the RTC running and the 'cmos bios memory' backed up ?
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You probably can, but you'll need to add the diodes that stop the battery from backfeeding into the whole system and charging the battery and remove the jumper wire that currently feeds the dallas and replace it with a diode. (likely the one that stops the backfeeding the battery to the whole system.

The DS1287 has an internal battery that is not accessible at all via the pins on the chip unlike a DS1285 (in fact, the DS1287 is essentially a DS1285 with the battery pins folded up and connected to a battery + an oscillator crystal, which are sealed with a plastic cover). So there is no way to disable the internal battery and connect it to an external source without hacking the chip.

Now, it is possible your motherboard has a circuit that provides external battery power to the Vcc input of the Dallas chip (pin 24) via the connector you show in the picture. However, unlike the internal battery this would require a voltage in the 4.5 - 5.5V range, which means you need an external battery of either 6V + one or two diodes to lower the voltage, or 4.5V without any diodes. But the fact that the external battery connector and diodes are unpopulated on this board makes me suspicious, I think those are probably intended for other variants of the board that use RTC/CMOS without an integrated battery.

In any case, you should trace the external battery pads and see where they end up, some pics of the board may also be of help.

i didn't see the Dallas chip... (blind!...blind!...blind!...).

TheMobRules is right.So in this case it is easier to solve...

Mod the Dallas chip or buy a new one if available.

That's what I would do if the computer was mine.

Ok, i traced the external battery pins to the dallas chip and they make no sense. VCC is connected to Ground and some other pins don't add up either. I guess TheMobrules is right - there are probably more components missing or the header is for another variant of the board. I guess i don't want to tinker with it, since i don't want to destroy the board. I just wanted to see if there's an alternative to modding the chip.

*sigh*, so it's grinding down the Dallas chip again. Can't tell you how much i hate these things! 😁

Thanks for your help!

Or get one of these if you can find it in your country.

There are some people who make and sell it, for example "Necroware" i think...

Rolling wrote on 2023-12-23, 19:01:

Ok, i traced the external battery pins to the dallas chip and they make no sense. VCC is connected to Ground and some other pins don't add up either. I guess TheMobrules is right - there are probably more components missing or the header is for another variant of the board. I guess i don't want to tinker with it, since i don't want to destroy the board. I just wanted to see if there's an alternative to modding the chip.

*sigh*, so it's grinding down the Dallas chip again. Can't tell you how much i hate these things! 😁

Thanks for your help!

From what little I gather, there is a way to mod the mainboard.
Care to share what you learnt from tracing the board?

my guess from the photo of your board would be that you need to, at least, replace the jumperwire on position D6 with a diode, and add, at least, on other diode in, I guess, one of either D3,D4 or D5 to stop the battery from being charged when the system is on.

To get the system up and running i I just drilled the Dallas-Chip and soldered an external coin cell holder to the chip.

I don't know if that's simple, because D3-D5 seem to have different connections to the Dallas Chip. During tracing i found f.e. that even the plus pin on the battery header is connected to ground on the Dallas. At first i thought this is caused by the bridge on D6, but even without it, it's grounded. One trace also runs to an unpopulated capacitor. Maybe it would be quite easy to mod but i don't trust the board, it seems there are some other shenanigans going on.

Anyway - i'll keep this approach in mind and will try to have a deeper look into it. Thanks again for your help!

Rolling wrote on 2023-12-23, 23:10:

I don't know if that's simple, because D3-D5 seem to have different connections to the Dallas Chip. During tracing i found f.e. that even the plus pin on the battery header is connected to ground on the Dallas. At first i thought this is caused by the bridge on D6, but even without it, it's grounded. One trace also runs to an unpopulated capacitor. Maybe it would be quite easy to mod but i don't trust the board, it seems there are some other shenanigans going on.

On boards without Dallas type chips there is usually a circuit for switching the supply for the RTC/CMOS between the PSU and battery power when turning on and off the system respectively. This involves some parts like diodes, resistors, transistors and capacitors and all of that stuff was probably left unpopulated on your board since the DS1287 does that switching internally.