f11

Geoff, are you referring post #1 in the 993 thread titled "1995 OBD1 Diag tool" ? What did you do with this small circuit? Is it inserted between Andy's OBD-II/ALDL circuit board and the PC? Or is it used instead of Andy's interface board?

I read the whole thread, and it isn't clear (to me, at least) whether this is an additional piece in the interface chain or a standalone.

Rod

Red rooster

Rod ,
That little circuit is the interface !!
It has the pc comms port ( COM 1 ) on one side and the diagnostic socket K,L ,+12v and ground on the other.
Its really not much more than a voltage level converter .

Worth another look ??

All the best

Geoff

Laurence Gibbs

Thanks for your clarification John. So if there are any chaps out there with the know how!
One point on the transistor interface. Please be aware that that particular circuit has no real electrical isolation built in. So long as the circuit is built correctly and no errors or fault conditions (with the connections or circuit) occur then all is well. The opto-isolator design has a degree of protection built in(as it's optically isolated input to output) as does the Max or sipex chipped convertor. The risk i think is minimal but should I think be pointed out, as for many reading the thread it may not be readily obivious.

Red rooster

Laurence,
Good point !
As some kind of reassurance both the K and L line DME drivers are designed to be able to withstand permanent connection to either ground ,0volts or battery, +12 volts. So even if you screw up a little on the build ,non operation is about the most serious side effect !

Thanks for all your efforts on this project.

Geoff

kristianw

I just downloaded the software and it looks great.. just need to find the hardware now. Would something like this cable work?
http://www.ecufix.com/shop/index.php...roducts_id=181

raycm

Isolation from what problem? The auto is not connected to the earth or any other electrical circuit. The 993 interface ties the 0 Volt lines together. There is no voltage difference between car and computer. The computer + Voltage is not directly connected to the auto. If you use a laptop on batteries my circuit is best. Two floating circuits tied together. No worry about static discharge. OK, it should have a fuse in the +12 Volt line.

If the auto is struck by lightning the transistors and resistors of the interface will be the fuse assuming the computer is tied to AC power and the AC earth is tied to the computer 0 Volts. All these conditions, of course, are extremely unlikely. Most devices plugged into your computer use no isolation. HDDrives, Modems, etc.

With all that being said, if I was to design a commercial 'for sale' interface, best design would use isolation and RS232 spec circuits. But even in this case isolation IC's have a maximum voltage differential spec. i.e. not perfect protection.

Ray

kristianw

Okay, after reading through the thread more i realize my last question was a dumb one. So, I'm wondering if this is the right thing. http://www.aldlcable.com/sc/details.asp?item=aldlbare Thanks a bunch to anyone that can shed some light on this for me

Lorenfb

"Isolation from what problem? " - raycm -

Given the multitude of posibilities, someone in someway will find away to damage something!
To think otherwise is rather naive, especially when using jumpers and/or non-standard interface connections!

Pesty

Kristin,

I think this is what after. you need an interface board first. A laptop with com 1(although there are other ways) have a read of teh first few pages again.
http://www.ecufix.com/shop/index.php...roducts_id=183

Laurence Gibbs

Ray i was'nt having a pop at your design! I think it's excellent. I was trying to point out to those who would not neccessarily know (there are a lot of people doing this that are'nt electronics people!) the "potential" problem. As I went on to point out, the risk IS minimal(if not virtually impossible, i have since learn't). As you point out yourself if you were building a commercial design you'd include isolation. Some people building your unit may not be aware of the difference and that was all I was trying to point out. I was thinking of possible damage to the dme from the circuit shorting out or other fault condition. The DME I now understand has some protection of it's own from shorts etc, so the risk is far far less than i had thought. With the opto circuits if a fault occurs on the rs232 side of the circuit the iso side would not be affected due to the electrical isolation of the two circuits. With your circuit this is not the case and it was this i was pointing out.

raycm

Hi Laurence,

I must have typed harshly. :-) But my writing lacks lots. I though about isolation when I did the design. I couldn't figure out why it would be necessary so I left it out. The world connects the PC serial, parallel, etc interfaces to other stuff without isolation. The IEEE specs do not include isolation. The automotive industry has a much harsher environment to design for. I'd guess the DME OBD interface is the strongest. The interface board the most likely to fail, for abnormal conditions. By design. :-)

A commercial business worries about law suits, public image, which allows adding some 'in case of' cost to product designs.

Pick on my design. Needs a fuse in the 12 Volt battery. Needs -V for the RS232 interface. But I know most RS232 designs work without -V input level. The transistor values are seat of the pants, I was too lazy to bother looking up beta figures. I did put in reverse diodes on the base circuits. And I don't know the OBD interface electrical specifications. I based this part on what other designs used, and scoped the resulting levels. Nothing worst case in this design.

Ray

kristianw

Okay, thanks pesty.. Could someone please send me a schematic of the interface if you have one?

Thanks,

Laurence Gibbs

Fair enough Ray:-).
Kristian, interface schematics can be found in the software zip for the maxim/sipex chipped desgn(thanks to Greg Kricorissian) or for the transistor design do a search on the 993 forum for diagnostic tool. Maybe ray could cross post it here too? The ecufix design comes with a schematic upon purchase. There is also the Jeff Noxon design, do a search (not tested though). I have also written a FAQ but it has to be searched for as it can't be stickied.

raycm

https://rennlist.com/forums/993-forum/271313-1995-obd1-diag-tool.html
The first message has the transistor schematic for the interface.
This circuit works on 964 and 1995 993's. The pin numbers for the vehicle interface are for the OBD style connector. Someone in 964 forum has built the circuit. Perhaps he can give us the pin out for the round style 964 connector, and jumper settings. I'll update schematic with this info.

Ray

Red rooster

Ray,
The the 964 OBD1 round connector,

Ignition switched +12 volts Pin 13

Battery +12volts Pin 12

K Pin 8

L Pin 7

Ground 0volts Pin 10


This is a great thread thanks to Dougs software. The OBD2 specification calls for a 510 ohm pull up on the K line. It may be that on some cars the 3.3 k needs to be reduced ?

As someone who is involved in " talking to DMEs " I really liked your minimalist circuit !!
I have a Hammer but think that this whole 964 diagnostics area needs opening up to all !After all its hardly current technology !!!

Good luck to all involved

Geoff