H.den Boer wrote:



For this pattern generator I need the dump of the main prom (D313).
It's a
27C256 with PLCC32 housing in a socket, located in the right corner
on the
back of the instrument. Can anybody help me?

What's wrong with the one currently in it? Have you tried reading the
data
from it with a lowered VCC?

I can read it, but databit 6 is always '1' the pin is shorted to Vcc,

Ouch. Well, reading an EPROM at a lower VCC will sometimes allow you to
get all the data off a flaky device, but in your case, it probably won't
help. Did you actually measure the short right at the pin with a DMM, or
just assume it was shorted to VCC from the data pattern?

It measures under 25 ohms, so no luck there, the prom is fried anyway,
it gets warm and consumes
too much power. It's a wonder I can read it at all, but without bit 6 it
is no good.

Does the supply current vary during read depending on the stored data?
If so, you *MAY* be able to retrieve bit 6.

It is a little difficult to be certain when you don't know what it
should be, but if you stick a sense resistor in the VCC supply (maybe
with two diodes across it to prevent excessive drop while switching) and
scope that and the read signal while reading the chip, you may find you
can tell the difference when the lower MOSFET in the D6 output stage
switches on to attempt to output logic 0.

If so, a PNP transistor, a few resistors and an inverter (possibly with
a tristate output controlled by the read signal) may be enough to build
a 'gimmick' jig to let your programmer read it.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
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