[Techtoolslist] realtime clock on Fluke 9100A/9100FT

[Tony Jones]

Not sure if this is useful info .... I'll throw it out there for the
archives ;-)

Both the RTCs on my 9105FT and 9100FT went bad not long after I got
them.  The original is a Dallas 1287 (same as the IBM PC).   It is a
discontinued part.   I bought a pair of 12887As from eBay which is a
commonly used modern day "replacement". The 12887 has two banks of
addressable memory (128 bytes vs 64 in the 1287) and the A variant has
a RAM clear pin.

Anyhow, neither 12887A worked.  The system would constantly report at
boot that the RTC battery needed replacement,  this seems to occur
when the 9100 reads null (epoch) data from the RTC.   The 9100 is able
to tell if it's a new chip as at first boot it instead reports that
the RTC is uninitialized.

I was floating the RAM clear (per datasheet), so my guess is the
second bank was the issue.  So it's possible a 1287A would have
worked.   There is an appnote #503 from Maxim addressing this issue of
possible incompatibility.

Anyhow, I purchased two 1287s from eBay (china).  I was a bit worried
about a fake as it's an obsolete chip but they both worked fine.

There is also a tutorial out there on breaking open the 1287 packaging
and adding an external piggybacked battery.   I just desoldered and
socketed and went for a replacement chip.

HTH

Tony