View Full Version : CMOS replacement

July 16th, 2009, 03:32 AM
I've been trying to get a working 486 system together in order to demo some old programs I wrote many years ago.

A colleague has given me a lovely 486 dx4 100 which I';m having trouble booting.

Naturally after 20 years in the attic the CMOS battery is dead. It's a Dallas DS1287A RTC module.

Will it recharge if I leave the computer on for a day or so?

I could buy a replacement but wouldn't these be dead also?



July 16th, 2009, 03:38 AM
i havent had a dallas battery go bad, since the one i have is unplugged. however the coin cell betteries, when they go dead, they are dead. i would send you my dallas battery though i'm not sure yours and mine (a DS12B887) will interchange. plus i'm in USA and you're in UK so there would be some hefty shipping

July 16th, 2009, 04:11 AM
The Dallas modules contain a lithium coin cell, embedded in epoxy in the upper half. Once it's dead, it's dead, and it won't be coming back.

The module has a date code on it. So, when looking for a replacement, try to find one that's as new as possible, giving the best chance of getting a good one.

I have not done any hacking on the Dallas modules, since none of the machines I run use them, but I've got signifigant experience with the ST Timekeeper modules, as used in Sun workstations and similar. Rather than replace the entire module, I cut into the existing one with a dremel and a knife. I cut the connections to the existing battery, solder wires to the chip, and then to a standard coin cell holder. Note that you have to be VERY careful you don't cut into the battery itself, or damage the module.

The top of the ST module, and I would assume the Dallas module as well, contains a lithium battery in one half, and a crystal oscillator in the other half. You need the crystal to stay intact and connected, but the battery needs to be disconnected.

But, since I don't know how the Dallas module is constructed, I can't give instructions on how to go about cutting into it. But, a quick bit of searching online turns up this: http://www.mcamafia.de/mcapage0/dsrework.htm

Looks like it's even easier than the ST modules I've been working with. Much larger contact points inside, and much simpler construction.


nige the hippy
July 16th, 2009, 11:18 AM
pop me a PM with your address on, I can pop you a ds12887 (the new replacement) in the post.
I have a tube of sockets sitting next to me too, if you're polite....;)

July 16th, 2009, 11:43 AM
I have done the dremel trick to several Dallas chips (on Sun, SGI and PS/2 systems) and it works quite well and sometimes can be a heck of a lot cheaper than locating a replacement chip.

July 16th, 2009, 12:52 PM
Even buying a new-ish replacement is only going to delay the inevitable. Weren't there some devices made with removable "top hats" instead of the "mystery in a block of epoxy" style? Next best thing is to fit the dremel-ed old one with a set of leads or a coin cell holder.

July 16th, 2009, 01:51 PM
... I've got significant experience with the ST Timekeeper modules, as used in Sun workstations ...

Hmm, my new-acquired SparcClassic seems to need a fix in this department too as it's 17 yrs old now and still on the original RTC. It boots to the bios and so I wrote down the mac addr, etc, while it's still present but sometimes this message comes up "eeprom! eeprom? eeprom@". Must be like tora, tora, tora...

So, to sirmorris pardon the intrusion and hijack but I was planning to simply purchase a new M48T08-100PC1 chip from Digikey but am wondering if the Sun non-compatibility issue that applies to the other sizes of later productions of this chip would surface? If so, I'm comfortable with doing the battery replacement surgery if required. I replaced the RTC on my IPC without fuss 9 years ago but must have got old stock.
Also, there is a plastic base around the chip with little tabs at the ends. Is that to remove the chip or is it a fixed part of the socket?

July 16th, 2009, 03:20 PM
Also, there is a plastic base around the chip with little tabs at the ends. Is that to remove the chip or is it a fixed part of the socket?

The plastic base around the chip is simply there to aid removal from it's socket, as well as to prevent inserting the chip backwards. It's actually keyed, and only fits into the socket one way. The chip itself just sits in this carrier. Once you've removed the carrier/chip from the computer, you can just pull the chip up and out of the carrier. Be careful to note the orientation, since there is nothing preventing you from plugging the chip into the carrier backwards.

The chip itself is made of two parts - the chip on the bottom, and the 'backpack' on top, which contains the oscillator crystal and the lithium battery. The backpack is attached on the short ends of the chip.

You'll notice on the short ends of the chip, there's a little 'dimple' filled with black epoxy, right under the plastic 'backpack'. You want to dremel into the 'dimple' on the opposite end of the chip as Pin 1. Go carefully, until you find the two wires, and expose them. Then, cut between the backpack and the chip itself, so as to sever the wires, disconnecting the old battery. Solder wires to the exposed contacts on the chip. The contact on the same long side of the chip as pin 1 is ground, the other contact is +3v. Connect them up to a coin cell holder and glue it to the top of the chip.


Anonymous Coward
July 16th, 2009, 04:58 PM
I just did a modification to an ST Timekeeper (Dallas 1287 compatible) chip. It was my first time, and I didn't find it a very enjoyable exercise. The ST chips are are lot easier to work with than the Dallas chips. For some reason the tin legs on the Dallas chips are really thin and easy to snap off. You should be be grateful yours is socketted too. On several of my boards it's soldered directly to the PCB.

One word of advise about gluing the coin cell battery holder to your chip, make sure you use something that adheres to plastic well.

July 16th, 2009, 06:51 PM
Thanks for the tips on the chip surgery. Will also buy a new one just in case it works out of the box.

July 16th, 2009, 07:40 PM
All the DS12887s I've got to replace the 1287s have worked fine, regardless of what I've put them in, PS/2s, Commodore PC-40-IIIs, clone 486 boards, everything.

You can get them for 5 bucks US each and the last time I got some, 3 cost me about 4 bucks US to ship from California to Canada.

The modules measured 3.06 VDC, out of circuit and everything they are in still works fine.

However, if you want to hack and grind plastic and epoxy just to get something solid into your lungs, knock yourself out.