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View Full Version : The Dallas Real time clock chips, questions



oblivion
December 11th, 2014, 09:15 AM
been looking at a few Motherboards with these chips and just have a few simple questions.

1) there just a form of CMOS battery correct? so if I had a board with a dead one it will still post and run just not save BIOS information?

2) read mixed things about buying replacement chips. will newer replacement chips bought off Ebay work or not? do I need to do that mod to use a coin battery on them?

Timo W.
December 11th, 2014, 09:49 AM
1) It's a chip combining RTC+CMOS, yes. If the battery is flat, the system may not boot at all, as it won't remember the hard disk parameters after POST and hence won't find a boot device. None of the old PCs I own from around 1993 was able to boot after the Dallas chip died.

2) If you're buying a compatible one, there's no reason why it wouldn't do as a replacement. You need to make sure that the chip is really new, however. If the date code on the replacement chip is already 10 years or more ago, you're just wasting money and better off cracking the old chip open and solder a coin battery holder in place of the old battery.

Chuck(G)
December 11th, 2014, 09:51 AM
1) depends on your BIOS. Some won't save anything if the battery is dead, even if the system is still powering the chip.

2) If by "newer" you mean a newer manufacturing date, of course that's better. Older ones depend on two things: (a) have they been previously used? and (b) how long has it been since the date of manufacture?

Previously used chips are chancy, since they've been draining the battery running the internal clock. NOS chips older than about 10 years are also chancy--lithium cells do have a lengthy, but definite self-discharge life.

sergey
December 11th, 2014, 10:07 AM
been looking at a few Motherboards with these chips and just have a few simple questions.

1) there just a form of CMOS battery correct? so if I had a board with a dead one it will still post and run just not save BIOS information?


Dallas RTC modules that were widely used in early to mid 90's consist of three things: RTC + NVRAM chip (it was made using CMOS technology for low power consumption), a lithium coin cell (CR1025 type?!), and a 32768 Hz quartz resonator. The battery eventually will get discharged, and the module will stop keeping data and time.

Most AT clones will work fine with discharged batteries, expect that:
- They will show an error message on boot and asking to press F1 to continue
- BIOS configuration won't be preserved. This could get annoying, e.g. you might need to re-enter your hard drive geometry on every power on.
- Clock won't work. DOS will still do it's own timekeeping, it will just not save time across reboots and power on/off cycles.
Some EISA motherboards might have more trouble with not working CMOS memory, as they save the EISA configuration there, and might need to run a utility to get it set up.



2) read mixed things about buying replacement chips. will newer replacement chips bought off Ebay work or not? do I need to do that mod to use a coin battery on them?

And rightly so. You might get an already used chip with drained battery, or a 20-years old "new old stock" module (although self discharge of lithium coin cells is low).

You can order new DS12887, DS12887A, or DS12C887 modules from electronic distributors, e.g. Mouser. They are not that expensive (about $10 each), and generally are compatible with DS1287. Here is a copy of an old Maxim/Dallas Application Note (http://www.100y.com.tw/pdf_file/MAXIM_DS12887A.pdf) regarding RTC replacement.
Or you can hack existing module, disconnect it's internal battery, and connect a CR2032 holder on top of it. This process has been described on the Internets several times.

Malc
December 11th, 2014, 10:12 AM
Just last week i bought a "new" ? DS12887 RTC for my GA-586 HX Board from ebay, It was only 2.50 so i grabbed one, It works and keeps good time thus far but only time will tell how long it lasts, About 6 months ago i did the battery mod on my Amstrad 486 box because at the time i couldn't find a chip under 20 and wasn't going to pay that much for one, It's not pretty but it works well.

modem7
December 11th, 2014, 11:16 PM
1) there just a form of CMOS battery correct? so if I had a board with a dead one it will still post and run just not save BIOS information?

1) depends on your BIOS. Some won't save anything if the battery is dead, even if the system is still powering the chip.
And sometimes, you cannot even change the settings. An example is at [here (http://www.vintage-computer.com/vcforum/showthread.php?41716)].

KC9UDX
December 11th, 2014, 11:30 PM
I once had a machine, that if memory serves was an XT clone, (maybe AT, don't quite recall) that would ONLY display an error message about a dead battery, when the battery was dead. It would not boot, nor allow me to get to the setup.

Also, now that I think of it, I have an AT clone currently that will only get error beeps on power up if the battery is dead. It will do nothing else.

If you must install a battery holder and/or battery in a way that the computer didn't have before, PLEASE don't install it near the motherboard, for future's sake.

oblivion
December 13th, 2014, 12:05 PM
The board I bought is a later board, a 486 shuttle hot-433 that has the RTC but I have some old ps/2's that have them as well. thanks for the info guys.