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 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.