10 May
2012
10 May
'12
12:53 p.m.
There was some debate at VCF East whether this "IBM 256KB Memory
Card" P/N 6407740 is a bubble memory card (pic below). The card was
on display with my IBM PC 5150 exhibit. I originally pulled it from
an IBM 5155 portable.
http://www.vintagecomputer.net/ibm/5155/ibm_256KB_memory_card_6407740.jpg
Typically you don't see 32-pin 64K RAM chips on a ISA card. That
does not make it bubble memory though, so I put this out to the
esteemed CCTECH community for answers/ideas.
I doubt very much if it's bubble memory, for sevrral reasons.
1) What would eb the point of having part of the sytem RAM non-volatile?
2) Buble memory is slow, and sequential access at the device level. It is
not really suitable for use as main memoty. My experience of it is
limited to the HP bubble memroy card used in HP9000/200 machines where
it's addressed in the I/O space and is used by the system as a fast 'disk
drive' .
3) Unless those metal cans are bubble memory devices with built-in
cotnroller circuitry, thwre is nothing apporximatign to a buble memeory
controlelr on that board. You can't do it with a handful of TTL parts. A
'bare' bubble memroy device has not that many conenctions either.
I am pretty sure there's a photo and schematic for the HP bubble memory
board I mentioned over on http://www.hpmuesum.net/
I do not have a technical reference manual, but I do have the IBM
64/256KB Memory Expansion Option manual. That ain't it.
That board is not min my Techref either. But I think it's just plain DRAM.
I do wonder why it existed. It is trivial to put 640K on the motherboard
of a 5155, jsut as in a 5160 (XT). It's a matter of putting the right
TAMs in (41256s in banks 0 and 1, 4164s in banks 2 and 3), adding a
74S158 (or 74F158) in the enpty socket towards the front and jumpering E1
to E2.
-tony