Interfacing OBD-1 to RS232C (via MAX232)

[markrbarker]

Hi, I've searched around the Forum for quite a while, but I hadn't been able to find an answer, so here I am asking for help please. I'm a newbie.

EFILive Eval Version, Win98, laptop pc c/w serial port

Question: Is there a schematic available to make an interface between OBD-1 and a standard RS232 port?

I understand a MAX232 is involved, that's OK, but I'm not completely understanding a loopback function going on that's required by EFILive, otherwise I would use this interface as a basis: http://www.techedge.com.au/vehicle/aldl8192/8192hw.htm

I can see from the above circuit a loopback going on at pin 'M'

Is this circuit suitable ?

Or should I really ought to be trying to buy this hardware from EFILive online store? :-)

[joecar]

Hi Mark,

I moved this to the V4 (OBD-I) section.

I pm'd Tech Support for more info.

Cheers
Joe
:)

[Tordne]

The link you posed (http://www.techedge.com.au/vehicle/aldl8192/8192hw.htm) is correct, and is actually noted on the EFILive V4 product page on our website :)

We also recommend this cable (http://www.moates.net/product_info.php?cPath=49&products_id=127) from one of our resellers, Moates Ware (maker of the RoadRunner PCM).

[markrbarker]

Many thanks for link for recommended cable (http://www.moates.net/product_info.p...roducts_id=127) I see it's got a mode selector '10K' switch included, and it's a USB interface which is great for laptops without COM ports as many are nowdays.

But I'm afraid I couldn't wait for one of these to be mailed to me, so yesterday I'd built myself a cable on a bit of matrix board, based on the above circuit. The EFILive software seems quite happy with it, there are no loopback errors.


I'm having trouble now with missing and/or incorrect data, I think might be a configuration error in the program maybe? I can detect car's ECU data telemetry (as far as I can tel) but there is a complete absence O2 signal displayed anywhere. I measure the mV accross the O2 sensor, it's there OK. I've also tried injecting a 500 mV signal across the O2 sensor itself, it's not picked up. Maybe the wrong TM byte is being addressed.


Also engine rpm indicator (dashboard) has a large error:

actual vs indicated
500 821
800 822
1000 823
2000 827
3000 832

Another thing I'm missing is interrogating DTC's. I believe they are retrieved in real time via the 'get engine data' macro

But apart from that it's fine...

Car's a UK Daewoo Lanos 1997

[Tordne]

Are you using the evaluation version (or the full version)? The evaluation version has some limitation (like not being able to save logs, showing limited DTC codes and logging restricted PIDs etc). I would not expect this necessarily to effect data accuracy however.

Where has the vehicle definition you are using come from (i.e. is it known to be accurate for your vehicle)?

If you can send your full VIN through to support@efilive.com I can see if we have a datastream specification file I can send you to aid with making an accurate vehicle definition?

[JezzaB]

I believe there is an error in the TechEdge diagram. One of the diodes is around the wrong way (D3)

[markrbarker]

Are you using the evaluation version (or the full version)?

EFILive Eval Version, Win98



Where has the vehicle definition you are using come from (i.e. is it known to be accurate for your vehicle)?

If I remember rightly it came already bundled with the Eval version software. I didn't have to look for any separate definition files.



If you can send your full VIN through to support@efilive.com


Yes, will do, thank you :-)

[markrbarker]

I believe there is an error in the TechEdge diagram. One of the diodes is around the wrong way (D3)


It looks to me like the pin M data is bi-directional wired-or (active low). Maybe an open-collector is at the ECU end, and the MAX232 pin 12 / D3 is a psuedo- open collector? If this is the case, I expect D3 to be correct polarity.

[Tordne]

Got the VIN. Sorry I hadn't noticed in a post above that your vehicle is a 1997. The data we have only goes through to 1995, so I don't have anything I can send you sorry.