I pulled out of storage my old Pentium MMX 166, and unsurprisely found that the RTC battery was dead. The message was "CMOS checksum error loading defaults" + "cmos battery failed".

I opened the pc up and found it has an RTC labeled VIA VT82887. Which I believe almost identical to a Dallas DS12887. The difference is the clear RAM pin as I understand it.

I ordered through eBay a replcement RTC (from China) being a DS12887A+. When I installed it I it did not work, CMOS checksum error...

I've since modded both RTCs to use a coin cell CR2032 from information I found on the internet. Now the Dallas does not give me a dead battery message, but just "Checksum error". I've checked clearing the RAM using the jumper on the motherboard, but that does not change the problem. I've also tried the modded VIA, but that still gives me cmos checksum error and battery failure.

Now I'm at a loss now to proceed with both RTCs. The only thing I think I can do is to try either Necroware's NW12887, or nwX287 RTC modules. I'm a bit unsure of using these, besides which one to order, as I do not know which is compatible with VIA VT82887, as it is not listed. I have read somewhere about compatiblity between DS12887 and VT82887 e.g. "The Houston Tech HT12888, HT12888A, and VIA82887 are not compatible with Dallas clocks".

Any suggests, or help, about any of this is much appreciated.