Thread: OBD-II connected all the time?

I don't think the is a problem in keeping it all the time.

Car with trip computer already reqading from ECU all the time.

Yes, but that is something designed in versus the service port. Trip computers likely bypass the OBD system entirely.

I don't know, just throwing that out there.

I'm not expert but logically no problem it in

Originally Posted by reece146

I was talking to a vendor about my project and they stated that their ODB-II cable is not meant for "permanently connecting" a car PC to the OBD-II port.

It all depends on the quality of the wiring, the interface, the software, and the car. Lousy wiring can act like a giant antenna that injects noise and corruption into the network. Lousy tools will do the same thing, or worse they carry voltage spikes and blow out your PC. Lots of cars are robust, but some cars are so touchy that anything out-of-spec pushes them over the edge.

Other vendors will stand by their equipment for semi-permanant installation, rough conditions, etc. Look around a little more before you settle on a solution.

Originally Posted by reece146

I can see the point regarding the whole baud change negotiation thing at initial but...

The ECU should spend its time running the engine and refuse commands or delay responses when its busy. Of course there are a few buggy ECUs as exceptions to the rule (often solved by a flash update!). BUT.... If you both you and the ECU follow the protocol, distance limitations, and electrical specifications then it will work just as well as it was designed to work.

Generic OBD-II has no negotiation and fixed baud rates. You may be thinking of ISO which has an unusual init sequence (a few bytes at 5 baud) and drops out of diagnostic mode with 5 seconds of no activity. Seems overly cautious in hindsight.

Originally Posted by reece146

Is the vendor just being overly cautious or is there a type of cable that is meant for permanent connection? Some kind of a buffered assembly that keeps the conversation with the ECU/PCM "clean" for the PC?

He knows his equipment best. If it's a cheapie solution its possible that they cut corners or don't spend the time to formally test and validate things. I really can't argue with somebody who says "don't use my equipment for this"

There are a million ways to break the rules so there's no buffer, filter, or magic fairy dust to stick in the middle and improve the situation. My only thought is to use different equipment and software where the vendor stands by the solution for semi-permanent installation.

Yeah, my setup is ISO9141-2 with the wonky init...

Here's a published listing of what is supported by the software:

http://freediag.sourceforge.net/Supp...html#supported

Maybe the Multiplex Engineering "opto-isolated" interface would be a better choice for permanent mounting? I dunno - just throwing that out there.

Lot of fuss to get the ALSA master volume bumped up higher based upon vehicle speed, eh?

It is a lot of fuss, but I bet it'll be cool when it's done

Optical isolation is always a good thing so you don't risk blowing out the PC's serial port. If you're going to do any custom programming consider a real PassThru (SAE J2534) interface instead of hobbyist equipment. Other than that it's same as anything else -- go with a real company that you can reach by phone with business hours, not just email or a web store.

Good luck. Post your results! Do you have steering wheel volume controls? It would be cool to tap into those if they're on a network.