Old MOS Mask-Programmed ROM forgetfulness?

Rick Bensene rickb at bensene.com
Mon Feb 15 12:25:51 CST 2016

Hi, all,

I have a question about old Mask-Programmed ROMs

The part in question is the National Semiconductor MM5231.  This part is
a 2K-bit PMOS Mask-Programmed ROM, generally organized as 256x8, but
also can be organized (via a MODE pin)as 512x4 bits.  In this particular
application, the parts are used as 256x8.

I'm wondering if anyone knows if these particular ROMs (from the '72
timeframe) have a tendency for bit rot over the years?

I know some of the early MOS ROMs had issues with metallization creep
that would cause data loss/corruption.

I have an old calculator that uses these ROMs as the micro and macrocode

The machine is catatonic, though the power supplies, master clock
oscillator and divider circuitry, and the other obvious stuff are  OK.
I suspect it is probably stuck in some kind of microcode loop, just
cycling around doing nothing.  I have not yet put  logic analyzer on the
microcode latches yet, but that's probably my next experiment.

Sadly, if one or more of these ROMs (there are 18 of them!) has failed,
it likely means that the machine can't be restored to operation, as this
is quite a rare machine, and there just aren't many of them left around.
I have three different EPROM programmers, but sadly, none of them have
the capability to read these parts.   I was I had a Data I/O programmer,
but alas, haven't come across one with all the Unipak modules I'd need
at a price I can afford.

Rick Bensene
The Old Calculator Museum

More information about the cctalk mailing list