No sumcheck code in a '95 MEMCAL

[Ludis Langens]

Roger Heflin  <rah at horizon.hit.net> wrote:
> There is not other rom on the board, Motorola has one in the CPU's
> themselves, and there is the possibility that the car comes up with
> the other rom, checks the socketed rom and resets into that rom with
> the CPU rom disabled.  That is something I could see them doing.

The older C3 ECMs don't have ROM in the CPU.  (There is ROM in an ASIC,
but that is in addition to the PROM, not a replacement.)  I can't (yet)
say for certain, but the P4 generation looks to have a ROMless CPU also.

> My
> car will run with a bad rom (read bad checksum, SES light) which would
> imply that it may run without a rom (it runs really really bad), which
> would imply that there is code someplace else running things.

Not code - but an entire extra analog computer "programmed" with the
resistors in the MEMCAL.

> I will have to see if I can find that code.  I have not found any
> loops that run through all of the rom (or a large part of it) yet.

If your ECM is a P4, look at all the places that write $FF00 to location
$400B/$400C.

> I wonder why they check the rom on reset?  Do they think it is that
> likely that it could have went bad? or are they just covering all of
> the bases

It has been standard practise to checksum the ROM in computers since the
IBM PC.  I think I have seen a ROM failure or two (in personal
computers, not ECMs.)

EPROMs have a limited guaranteed data retention lifespan - something on
the order of 10 years.

> finally decided that they had not had any of the type of
> problems so quit doing it?

Two ideas, both having to do with switching to OBD II style error
handling:  OBD II may require the CPU to try running even with a damaged
PROM.  Or, while the engine is running, OBD II may require the CPU to
recover very quickly from a hardware (i.e. watchdog) reset, leaving no
time to sum the PROM.

> I guess you might try putting a bad checksum in there and see if it
> notices.  If it does, it would seem to be taking care of it somewhere.

Difficult at the moment.  The engine&ECM in question have no vehicle to
call "home" at the moment.  Plus, I think the engine's owner has it torn
down to rebuild.

The checksum was already "bad".

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/