data logger files - software to read them

[alexpeper]

I am not sure about obd1 but GM obd2 uses manufactured defined data packet
numbers not memory address, offset or parameter number to
request data.  All models and years have different or same request numbers,
yet report back the same data and scaling.  Thus if you made a request
for data item 0812 rpm you would get water temp for one car and mph
for another.  You would need specfic data request tables for each car,
or GM could be using a decryption formula based on model and year.
This does not seem to be easiest way espically for the scan tool.
Ford request values apply the same to all years and models, thus if
you request rpm for one car you will also get rpm for another.
I have also heard that GM does same with some maunfactured defined trouble
code numbers.  where the same number will have different definitions
for different year and or models.
Alex

Date: Fri, 31 Mar 2000 09:56:50 -0600 (CST)
From: Roger Heflin  <rah at horizon.hit.net>
Subject: Re: data logger files - software to read them

The GM data encoding is not in any way shape or form and encryption.
It is simply a encoding of what data bits are what.  Encryption
denotes someone attempting to make the data hard to deal with, I don't
believe they did that, I believe the just encoded the data the easiest
and quickest way.   Their scalings are based on what ranges the
sensors have and how many bits they have to encode the data in.   THey
also have limited space for the error flags, so to make the messages
all be the same, they would need to have more bytes in the data stream
(alot of which are unused) and would have less frames per  second.

Roger





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