I know that there are several cable manufacturers out there that exist; AKM, ALDLcable, ... the list goes on. These cables are between $60 and $120 and I've read great things about them.

My question is, when we are talking about USB connectivity, what is different electrically between the following:
1. AKM/ALDLcable OBD1/2 to USB cable
2. USB to RS232 adapter available at monoprice/amazon

To me, it appears as though they do the same electrically. Communication between the OBD port and RS232 port basically just serves to invert the data coming from pin M/9.

Assuming I connect the correct pins of the DB9/DB25 connector from a RS232-USB converter to the correct pins on the OBD connector pin A for gnd and pin M for serial data, would this work?

Remember, as I have a 93, I am not concerned with data being transmitted back to the car because I cannot flash the ROM.

 

There's like 3 serial to usb converters that will work. Only certain FTDI chip based converters that support non standard baud rates will work.
Which one are you refering to? The interface cable converts the PCM's TTL to RS232, which is technically not even necissary when using a USB converter.

 

I was just referring to a general application. I know that the baud rate is slow somewhere around 1Kbyte/sec, but i don't know anything about the signal voltage levels. I've seen a schematic to convert ALDL to RS232 (2 transistors method), and have to assume that the voltage swings are very low if the need to reference 12V is there.

No one has had any luck with anything but FTDI chips? I noticed that ALDLcables uses the FTDI chip. To get a cable shipped from FTDI is almost as much as the ALDL cable and it doesn't have the connector on the end of it, it was about BP 22.5 or $40.

I was curious to learn more because I didn't find a lot of references to it anywhere on the web. What got me thinking was Shoebox's 95 OBD2 connector fix for an OBD1 scanner.

 

The PCM operates on 8192 baud, the closest standard rate is 9600.
I used AKM's MAX232 schematic and bought a Keyspan USA-19HS converter and it has always worked great. Total cost was around $45.

 

Awesome! Thanks for the feedback. Your creation looks marvelous! Did you try it before you added the AKM 12V switching portion?

My hypothesis is that this portion of the circuit shouldn't be needed with the newer chips. At work I've used RS232/422/485 chips that accept differential inputs (what is given by the data/gnd pins) with voltage differences as low as 0.2V and as high as 17V.

 

I was using an older laptop with a serial port. Then I got a new laptop and upgraded the cable to USB.

The FTDI USB adapter works with 0-5v inputs directly from the PCM (the other one I tested anyway, not the keyspan but still FTDI based). It was pretty finicky and probably required some buffer transistors to clean up the logic.

By the way, I believe RS-232 is -12v low, +12v high, with -3 to +3 being undefined. The whole point of the MAX232 chip is to convert the PCM's 0-5v to positive and negative logic.

 

It's not the baud rate that's the 'trick' in the cables, necessarily, but the tighter timing requirements for the PCM.

The PCM is 'interactive' when you're talking to it. Laptop sends a command, PCM replies back, etc. There are narrow turnaround windows involved. The FTDI chip (I use the FT232RL chip) works very well (especially when people don't futz with the FTDI driver timing parameters like has been known to happen ).

Buying a USB to RS232 adapter from Amazon etc is kind of a crap shoot. It might work, it might not. I tried a few from ebay (thinking "hey, I can buy a $7 USB adapter and there-we-go"). Nope, those didn't work for me either. I tried those 2-transistor circuits (serially speaking), even sold a few cables built that way, but a couple of those customers had issues with their laptop and those cables, so I stopped that. I stick with what works for me and best overall... the FTDI chips for USB, and the MAX232 for the RS232 versions.

You may find that the simple 2-transistor circuit works fine with your laptop. It did with mine. But keep in mind that as I sell these things as a little business, I do not want cables to come back home with a "it doesn't work" note attached. I go for "I know this will work, and I trust it" setup.

 

I'm not sure this was directed at me, as there is no 2-way communication as far as I know on my 93. I thought that there was a "bit sequence" that just kept repeating with different data in it, a constant stream of constantly updating data. The only real question I have now is if i can directly interface this http://www.mouser.com/ProductDetail/...3R5gliADBac46E to my car and computer.

What I do know is that the RS232 standard does define the signal properties exactly as MikeGyver said earlier. What I can't find is the GM spec that defines signal properties, or anything on the web that describes what the signal voltage levels coming from the OBD connector, i.e. if the output is ground crossing, and what the magnitude is.

I have no o-scope to verify that the OBD output drops below -3V, which I highly doubt. According to the schematics I've seen, the max output voltage is GND, not -3V. Being an electrical engineer, I feel like I have to understand everything .
http://4.bp.blogspot.com/_1PMLLkxJyo...s320/aldl2.jpg


Does anyone have info on the signal voltage levels coming from the data pin on the ALDL port? It seems like it might just be a 0-xV signal coming out. They use this to control the transistor that switches the output between GND and 12V.

Thanks

 

I scoped my data wire and it was 0-5v.

 

Yes there is.

Look at this document for the '93 LT1 on my web site:

http://www.akmcables.com/a172b.txt

You'll see the different "mode" commands that can be sent to the ECM. Those determine what data gets sent back. Mode 1 Message 0 is the message required to get the diagnostic data sent back from the '93 ECM.

The sequence would look like {chatter} -> send shut up command -> send Mode 1 Message 0 request -> {listen for diagnostic data packet} -> chatter resumes if you don't continue requesting Mode 1 Message 0.

And the ALDL signal wire is a 0 to 5 volt bidirectional bus wire.

As for the FTDI USB adapter, that one will work, as long as you bus the TX and RX on the RS232 side together.

See the transistor/resistor logic on my schematic here:

http://www.akmcables.com/aldlif.jpg

Wipe out the MAX232, put the FTDI TX/RX in its place, and there you go.

There are other versions of the bus side logic (using a diode/resistor network as well), that may work, but I've stuck with the transistor/resistor network.