Calculating the Checksum

Roger Heflin rah at horizon.hit.net
Fri Sep 11 20:33:03 GMT 1998


There is computer instructions that have to be run to calculate the 
checksum of the prom that is plugged into the computer.  If the checksum
is wrong you get an SES light (that is if the checksum is not disabled),
and the computer tries to run the car with the backup code that is 
already in the computer (it runs badly this way, I put in a prom that
was supposed to be for a 93 Z28, Summit sent it (two different ones 
actually, both failed), and the car does run just really badly).  The
code for the actual checksum needs to be running before anything in 
in th prom is used to validate that the prom is good, so that code needs
to be someplace other than the prom, since you don't want to execute the prom
until you can at least tell that its checksum is at least right, otherwise the
car won't start.  A completely bad prom would seem to lock the computer up
until the watchdog restarts it, and then it would lock up again, so
the checksum is a crude way of detecting an bad prom.  There may be
other bytes in the prom that also verify that it is a correct prom
for the computer to prevent an incorrect prom.  The checksum is to validate
that at least everything is the same as it was when the prom was programmed.

				Roger

On Fri, 11 Sep 1998 Don.F.Broadus at ucm.com wrote:

> 	Could you please explain  checksum for me.  I thought checksum was a
> hex value
> 	That was used to verify if an EPROM had the correct program  data.
> 
> 	
> Thanks  Don  
> 




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