NetNews Usenet Archive 1993 #1/NN_1993_1.iso/spool/rec/autos/tech/17750

Internet Message Format | 1993-01-12 | 3.4 KB

Path: sparky!uunet!spool.mu.edu!howland.reston.ans.net!zaphod.mps.ohio-state.edu!rphroy!albert!rhaar
From: [e-mail] (Bob Haar)
Newsgroups: rec.autos.tech
Subject: Re: GM ECM Serial Data
Message-ID: <[e-mail]>
Date: 12 Jan 93 15:03:42 GMT
References: <[e-mail]>
Sender: [e-mail]
Reply-To: [e-mail]
Organization: G.M. Research Laboratories
Lines: 61
Nntp-Posting-Host: albert.cs.gmr.com

In article [e-mail], [e-mail] (Bob Brickman) writes:
|>In article <[e-mail]>,
|>[e-mail] wrote:
|>>
|>>     "I want to find a way to read data from in into a computer.  (So far I
|>> can
|>> only get a readout with p's and x's.)  I have modified the signal output
|>> voltage to match the required input by my computer.  Also, I have been
|>> able to determine that the data rate is 2400 baud (with aid from an
|>> oscilloscope)."
|>>
|>> Bzzzt.  It's actually 8192 bits/sec.  (I'm assuming you have the "high-speed"
|>> P4 ECM.  The older C3 is 160 bits /sec.)  I've often wondered about the session
|>> where the GM engineers decided on these numbers.  What I wouldn't give for a
|>> few ounces of whatever it is they were smoking at the time.  I guess they're
|>> making progress though.  At least 8192 is a power of 2.
|>>
|>>     "Can someone tell me how many start bits, stop bits, and whatever the
|>> signal coming from the car has?"
|>>
|>> They can, but they won't.  Give up.
|>>
|>>     "(Any other information on this signal would also be appreciated.)"
|>..... etc.
|>
|>    I saw a couple articles in September 1992 "Automotive Engineering"
|>which gave brief descriptions of the SAE J1850 and J2178 standards for the
|>serial communications network and message format. Perhaps these may be of
|>some relevance to you. They still don't describe how to interpret the
|>content of a message packet, but it may indicate how much of a job you have
|>ahead of you if you want to "roll your own".  Buying a package from Diacom
|>sounds pretty cheap by comparison.

The Recommended Practices [not "standards"] will not help for
todays cars. They will start to appear on some vehicles in '95
to meet California's OBD II regulations. J1850 describes the
low level protocol - wave forms, voltage levels, timing, message
framing, etc. J2178 defines some standard message formats and header
usage. But there will still be manufacturer specific portions
that are not public knowledge. If you are interested in these,
the best source is the J documents themselves.

If there is enough interest, I can describe these protocols briefly,
as I am a member of the SAE Data Communications Committee that
is developing the protoocols. My involvment is in the protocol
being developed for real-time distributed control.

G.M. still considers most of the current communications protocols
to be proprietary. I have a number of internal documents, but am
not allowed to pass it on outside G.M. The Service Technology
Group does make some of this information available to manufacturers
of service equipment, but not to the general public.


---
      Robert Haar               InterNet : [e-mail]
    Computer Science Dept., G.M. Research Laboratories
DISCLAIMER: Unless indicated otherwise, everything in this note is
personal opinion, not an official statement of General Motors Corp.