[sdiy] [OT] puzzling EPROM problem

[Trevor Page]

Hi list,

I've got a funny EPROM problem here. Sorry for the OT, but this is probably
the best place to ask this sort of question.

I'm repairing two Martin Imagescan projectors (dirty great big heavy things
that spin strange images all over the place in nightclubs!!!). Without going
too much into detail, the projectors were failing to initialise properly.

They use an Atmel CPU and have an EPROM (27256) containing programme
information. The EPROM versions in these projectors was V3.0. Having spent
ages on these things and coming to a complete loss as to what the cause of
the fault was, for a laugh I tried reprogramming one of the EPROMs with the
latest image, V5.0.

That caused the projector to work perfectly.

Before reprogramming the EPROM, I verified it's contents against a V3.0 mask
that I downloaded also, and it verified correctly. So it's not as if the
EPROM contained bad data in the first place. V5 may well have bug fixes and
so on over V3 - but it's strange how updating the firmware like this would
make a suddenly disfunctional machine work again.

When an EPROM is programmed, or with the passing of time, is it at all
possible for certain bits, or 'fuses', to be only partially 'blown' so that
although the logic levels of those bits are such that they satisfy the EPROM
programmer that they're correct, these particular bits read out at funny
voltages when the EPROM is in use, and hence the CPU reads them wrong? But
then again, the outputs are surely buffered, aren't they.

The EPROMs are being accessed pretty fast. The CPU is on a 24MHz xtal. If
this means anything. They're 27256-10's.

One other possibility is that the CPU is detecting a certain fault condition
from one of the sensors, and whereas V3 would just hang up in the event of
this happening, V5.0 will carry on regardless. But there are really very few
inputs to the CPU - DIP switches for mode settings, a microswitch and an
optosensor - all of which work fine.

Any thoughts? It's bad enough not being able to repair something, but it's
worse having got something working and not knowing why...

Trev (sitting in a dark room, with psychaedelic images floating around the
walls and ceiling... :-)


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