On 30 Mar 2006 06:22:03 -0800, "ve3tjd" put finger
to keyboard and composed:

Hi Franc:

Thanks for your input. First, I would like to reply to Geos reply.
There is a 2764 right next to the SRAM, but I've tried to read the
eprom and it contains no data. On the working unit I have, I took
another 2764 (NOT THE ORIGINAL!!) and erased it (all FF's) and
reinstalled it and it still worked normally, telling me the code isn't
on the 2764.

If you remove the 2764, does the device still work?

What is the purpose of the 2764....I don't know.

Is it possible that the "dummy" 2764 could be a place holder for a
diagnostic EPROM??? Could such a chip be used to initialise the SRAM
with factory defaults??? Once the SRAM is initialised with functional
code, could the EPROM be replaced with an "all-ones" dummy that would
cause the uP to cycle through its addresses until it hits the RAM
code???

The uP isn't socketed, so grabbing the address and data lines from this
angle won't work. I socketed the broken units uP as I thought that it
was shot, so once I get the data OFF of the good unit, programming
should be fine.

From the data sheet of the 5518, they say it's a drop in replacement
for the 2716. Do you think the uP address and data lines would cause
too much of a load for the programmer to allow me to try and read it
while the uP is in place?

I don't know, but you would need to ensure that the uP was in reset
mode so that its address and data lines would be high-Z.

Why am I so worried.... well, these boards are no longer made anymore
and I don't want to kill the good one by trying to fix the bad one! No
schematics are available (the board was made by TRW in England), so
without reverse engineering the 4 layer board, I think we have to use
common sense with the layout of the "system".

The easiest thing to do would be to solder leads onto the SRAM while
it's in the board, and connect them to the programmer for
reading.That's why I ask if you think it will hurt to leave the uP in
place.

I'd experiment by reading and writing the RAM in the bad board.

Thanks again guys,

Tedd, VE3TJD

- Franc Zabkar
--
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