Help needed reading Prom.

[David A. Cooley]

At 05:03 PM 11/11/98 -0800, you wrote:

>> Hi to you all,
>> 	I have come across a 28 pin prom that I just cant get a stable checksum
>> on. When I read it as a 27C128 I get FF in the bottom half of memory and
>> what looks like data in the top half. With the prom in its own circuit
>> pin 28 (VCC) is joined to pin 26 (A13) which would explain that they are
>> only looking for data in the top section of the prom and why the bottom
>> section is all FF. 
>> 	The prom data lines "D0" to "D7" going to the MC68hc11 are all
>> reversed, "D0 goes to "D7", D1" goes to "D6" etc. This of course will
>> give the incorrect hex readings when the prom is read but what is
>> causing the checksum to be different on every read attempt?
>> 	I have made sure all the pins are very clean as I know from experience
>> this can cause different checksums.
>> 	Any help would be gladly welcome.

Richard,
This sounds like the newer chips the GM cars were using... Should still be
a 27128 or 27256, but they scramble the address and data lines to make the
code basically unreadable if you read it like a "normal" prom.
If you make an adapter to cross the leads like they are in the PCM, you can
read the code properly.
As to the Checksum changing, compare the BIN between 2 reads and see what
changes.  I had a similar problem with an MCT eprom burner... The
replacement Jameco sent me was the same... It conflicts with the PC's RAM.
Bought a Needhams PB10 for the same price and no more problems.

Later,
DAve

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           David Cooley N5XMT           Internet: N5XMT at bellsouth.net
     Packet: N5XMT at KQ4LO.#INT.NC.USA.NA   T.A.P.R. Member #7068
       I am Pentium of Borg...division is futile...you will be approximated.
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