Thanks for trying. I did try to wire up the circuit you posted in your first post, but I used
2N3904 transistors, but I couldn't get it to communicate with my laptop. Those transistors are only rated at 200ma. I don't know if that'* a problem. I was able to get it to get a checksum error occasionally, but never got a complete data transfer. Usually, with the wiring as described, when I send a command, it hangs until I turn off the key on the car, and then it returns empty, so I know it is responding, although wrongly. I measured the RX side of the laptop when it was hung, and it read -9v with my DVM.

2N3904s are fine; nowhere in that circuit should you ever draw anywhere close to 200mA of current. If you are getting checksum errors, I'd bet that your problem lies in timing and collision problems. I posted my settings earlier in this thread, but you may have to experiment with them, as good settings seem to vary from computer to computer. Also, try the "hush chatter" command I described in an earlier post before requesting the mode 1 message 0 data; you may be trying to send your request at the same time the PCM is chattering, causing a data collision and a checksum error or a program hang.

To troubleshoot the signals in the circuit, you'd really need an oscilloscope, and even then it'* kind of a pain; the signals aren't regular, so the oscope can't cope (unless it'* digital, and then you've got more money than I do )

-b

Well, I tried all your suggestions including the hush chatter. I will look at my nine pin connector some more when I get home. Maybe I had it wired wrong.

One other question regarding the laptop connection. You said to use the RS-232 connection. Is this what is commonly called a COM port? In my device manager window, it has parameters for the COM port, and I can select the baud rate up to over 112K baud. It also has some settings for flow control.

Yes, RS232 is, in short, your COM (serial) port. The settings in the device manager window don't matter, but the communication settings in CarBytes (Edit-->Comms...) must be correct:

Port: whatever
Baud: 8192
Data: 8
Parity: none
Stop: 1

The buffer defaults of 256 bytes should be fine; the timing settings you may have to tweak to match your setup.

On second thought, I don't know if CarBytes insists that handshaking be off or if it uses the Windows default settings, so you might want to check that port'* settings in the device manager and change the handshaking to "off" as opposed to XON/XOFF or hardware. (Handshaking is just a nice way of each device telling the other it'* ready to send, ready to receive, etc; but the PCM isn't that sophisticated, so either of those settings would screw with communications.) Device manager shows that my port'* handshaking is off, and I don't know if I've ever tried it with any other handshaking setting.

I'd try it now, but my car is dead. :? Damn upper intake.

-b

Doublecheck that your transistor pinout is correct; we've had trouble with that in the past. But I really do suspect a software / signal problem; a checksum error means that serial data *was* received; it just wasn't the data that the PCM sent. A circuit problem would probably prohibit any kind of data transfer.

I'll ask you the same question I asked... don't remember who, sorry -- how long is your cable, and is your conversion circuit closer to the car'* diagnostic connector or your laptop?

-b

I have 6 feet of twisted pair cable with a ground shield between the circuit and car, and another roughly 6 feet from circuit to computer. Is there a recommended cable to use. My cable is actually intended to pass balanced audio with around 30pf/ft. I thought it should work okay at these speeds, but maybe not.

Hell, that'* a lot better cable than what I'm using -- I've got 50' of unshielded twisted-pair four-wire telephone cable. That cable should be fine. It'* not long enough to make much difference either way, really. At 30pF per foot, and at about 8kHz, the shunt capacitive reactance of that cable is about 650kohm. You shouldn't lose any noticeable amount of signal with that cable.

-b

I thought it would be fine. I had alot of extra audio cable laying around. You don't think
havind the 12v twisted around the data signal is a problem? By the way, sorry about your intake problem.

enmityst,

I'm still brainstorming this thing as I'm at work. With the circuit you provided,
I calculate a maximium of 1.1ma flowing through Q2. One thing I tried when I
was troubleshooting was put an LED on data to ground and again from RXD to ground,
and the LED flashed brightly at the DATA side, but was very dim at RXD. Do you think changing R2 to a smaller value like 5K would hurt. How much current can RXD take? Since the transistor can take up to 200mA, there'* alot of room to play with. When I put LED on TX, I get a bright flash whenever I send a command.