Old MOS Mask-Programmed ROM forgetfulness?

Holm Tiffe holm at freibergnet.de
Tue Feb 16 06:51:33 CST 2016

Rick Bensene 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

I can only speak for similar east german made ROMs, both 256x8 and 1024x8.
The 256x8 (U501) part was pin compatible to the 1702A and the other one (U505)
to the 2708 and both of them (1st PMOS, 2nd NMOS) where much less reliable than
the compatible Eproms. In fact if I get a bad PCB with such ROMs I've
expect a failed ROM first.


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