Diacom, OBDII interface (was Re: Engine codes for '84 T/A)

[m_mcdonald at marx.ENET.dec.com]

>> Well, it [Diacom] acutally hooks up to the parallel port.  And the instruc-
>> tions imply that the software has to initialize the hardware molded into the
>> specialized cable.  Have not gotten around to look at what goes in and 
>> out of this cable yet, since I no longer need to home-brew an ALDL interface.
>> The only reason I can think of for there to be hardware in the cable is
>> so that my 4.77 MHZ laptop can still read in 8192 baud data.  I imagine
>> a faster computer can just pole the parallel port and read in the data
>> that way.

> Could you please explain more about this. I'm no wizard when it comes to
> parallel ports. What is meant by poling the port? And how do they adapt
> the parallel port to a serial interface? I bet the cable could be duplicated
> for a few bucks, and if someone can explain what it does, I'd be happy to 
> experiment on this.
 
> Markus

My advice is to pay Rinda Technologies $300 or so for a complete Diacom
hardware/software package that most likely does everything you want.

If you choose to do it yourself, then your first purchase should be
Paul Bergsman's book about controlling the world with your PC.  It has
excellent descriptions of interfacing via a PC's parallel port.  It also
has many code examples in, as I recall, BASIC and C and Pascal.  Your
second purchase should be the SAE book, code HS-3000, about standardized
OBD-II hardware and messages.  You've spent about $90 already and could
have dozens if not hundreds of programming hours ahead of you especially
if you have to learn about data communications.

Diacom does not "adapt the parallel port to a serial interface."  It uses
a PC's parallel -- not serial -- port to communicate with the GM cars' data
stream via the ALDL connector.

The Diacom cable could probably be duplicated for a few dollars, but the
software is the expensive part.  I'd spend $300 or so in a minute if it
were available for my 1993 Ford Escort with its EEC-IV data stream.  From
what I've observed, the Diacom package is an exceptional value.  Snap-on
Tools charges about $1,000 for its MT2500 hardware package that only
displays four lines of about 20 characters each on a small screen.

Marll McDonald   KB1AGM
m_mcdonald at marx.enet.dec.com