Old MOS Mask-Programmed ROM forgetfulness?

Pete Lancashire pete at petelancashire.com
Mon Feb 15 13:09:37 CST 2016

The ones you have to watch out for as a rule are those made by Mostek. Not
sure of the date when they changed to recipe.

If your still in/near PDX I will be getting my Unisite+ back in March and I
may have a working Model 29 that you can borrow.


On Mon, Feb 15, 2016 at 10:25 AM, Rick Bensene <rickb at bensene.com> wrote:

> 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
> stores.
> 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.
> Thanks,
> -Rick
> ---
> Rick Bensene
> The Old Calculator Museum
> http://oldcalculatormuseum.com

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