On Sun, 22 Jan 2017, Chuck Guzis wrote:
It's worth noting that the original post was about
replacement using the DS12885A RTC chip. However, most old PCs used the
DS1285 RTC (inside of a DS1287 module). The DS12885A is supposed to be
drop-in compatible with the DS1285, but apparently, in some cases is not.
Of course it's not, because the DS12885 has additional 128 bytes of
internal RAM addressable with the use of the AD6 line, which used to be a
don't-care for the address register in the DS1285. So if a system or a
piece of software uses bit #6 of the address for some other purposes while
letting it through down to the chip, expected to ignore it, then the
effect will be different with the expanded chip compared to the original.
Likewise with all the module variations made around these cores.
So there's logic in reworking the old DS1287
modules, as the DS1285 chip
is long out of production--you'll most likely have to be content with
pulls or the occasional NOS lot.
A problem with reworking is there is sometimes very little clearance
available, so any modification has to be made in a clever way or you risk
a short-circuit. For example in DECstation 5000 systems their DS1287 chip
is located in the TURBOchannel option slot area with the top of the chip
almost reaching the solder side of the PCB of any option plugged in there.