15 Jun
2010
15 Jun
'10
7:04 p.m.
I've spent a bit more time looking at the HP82909 128K RAM module for the
HP86/HP87 machines which I mentioned a few days ago.
I think I know what the missing chips are -- a pair of '30s, a '27 and a
'74. Thers's also a resistor (around 4k7?) and a transistor (TUN :-)),
not fitted as well. And a mode control link or switch which enables this
extra circuitry (and changes a couple of other logic signals that I've
not fully investigated yet)
if these parts were present, then it appears a write to location 0xFFC7
would enable or disable the internal system ROMs of the HP85/87 (bit 0 of
the data written determines whether the ROMs are enabled or disabled. I
don't know enough about the custom RAM controller IC in the module to
know if it can be programemd to map the RAM in place of the system ROMs,
or if a differnt controller IC is needed too.
I am wondering what this was designed for? Developing new versions of the
ROMs? Has anyone come across anHP product which could make use of this
capability?
-tony