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DIY OBDII to RS232


chrisjunkie
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It will work but it's a really old system and there are simpler solutions now.

If you really want to connect to the serial port, because your laptop doesn't have usb say, then you would be better off using a rs323 chip rather than all those reststors.

If you had a usb port then you can buy a lead with a rs323 chip built into the usb plug. They will run straight off as a serial connection to almost anything.

You will need software to go with that plug and most of the software out there is for use with OBD2 interperating chips, so they won't work with a serial connection. There is a limited amount of serial connected OBD2 softawae and most of it is fairly limited in the information it will gather.

If you have an OBD2 car it's better to buy a chipped lead made to interface between the software and car.

Have a read up on ELM 327 chipped code readers.

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thanks for the awesome reply! Not wanting to interface with computer rather with a pic or avr programmed to diplay stuff on an lcd. Will program this myself. Can i connect a max232 chip straight into the l and k lines? Looked at the elm chips but would rather not spend 30us if i can do it with cheaper chips.

Any more info would be aweome!

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If you're using an AVR (or other micro) you wont need the max232.. the AVR's built in 5v logic is what the ECU uses, you'd only need the Max232 if you wanted to USART with a PC. So just jack up the AVR's USART lines with the ECU ports and you're away (I forget the frame settings though.. 1 stop bit no parity from memory). One thing to look out for is the unusual baud rate.. you'll need to select your AVR clock and prescaler properly to reduce transmission errors

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Ah ok, so even though there is 12v available on the port, the k line is still only a 5v line? All im curious about is how do i send and receive? I mean TTL serial has 2 pins whereas on the obdii plug there is just the k line. Can you remember where you found this info or even the baud rates? My original post's link, is that the way to send and recieve on the k line? Haha sorry if I'm being real noob but im new to this obdii stuff and whatever I put into google gives me pretty shity results.

Cheers,

Chris

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I'm not 100% sure on OBDII as I only access the ECU using the SSM protocol (Subaru select monitor) .. to access the SSM you need your standard Tx/Rx lines at 5v TTL level (pins 12 & 13 respectively), and you can use the 12v power line to power the whole circuit. SSM is cooler, you can get more stuff and is the native subaru ECU protocol.

From memory OBDII uses half-duplex serial communication over the K-line (i.e. you send a request packet and then listen in on the line for the response) so you'll need to handle this in software or something. The K-line idles high and you can potentially tie Rx and Tx together in the circuit, so long as you use software to ignore the packets that you essentially send to yourself. Packets are 12bytes max (including crc) so you can easily write a routine to manage the half duplex comms.

Try googling 'SSM' or 'Subaru select monitor' .. it'll be more fun

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ah cool! SSM - completely forgot about that! Will have a wee look around now hopefully there is enough info floating around on teh interwebz..

EDIT: doesnt SSM only support newer models? RomRaider doesnt seem to support a v5/6 sti ecu

EDIT2: Will have a better look tomorrow. Bloody tired now and not thinking straight

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SSM works to way back in the early 90's.. I ripped my ROM out (94 legacy) using it. Just left a laptop in the car overnight with my max232 board and some shady app I wrote. SSM will give you access to every memory location available to the cars MCU, where as OBD2 is limited to RAM locations for sensors.. depends what you're trying to get I suppose! Helps to use some amplification on the ECU -> PC lines as the ECU struggles to drive the lines. Pull-ups help, but I often use a bastardised sziklai pair for the amp stage

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Hey chrisjunkie...

I have a TTL-232R (USB to TTL serial) cable here that i was using on my v5/6 ECU.

Log1call was helping me get it connected with evoscan.

We were very close but i had to get the car going with a Link ECu or the wife wud had left me :-X

I am happy to send it to you to borrow and if you get it connected and want it just give $20

(Log1call - if chris dosent want it and you know someone else that can assist you with your ECU missions let me know)

Let me know?

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 chrisjunkie said:

is SSM or OBDII TTL levels? Cant seem to figure it out

SSM is TTL.. here's the first setup that I used (before switching to USB). All that you'd need to do is remove the max232 chip and replace it with the AVR. This setup hooked in to the com port on a PC. I've just assumed a 5v source in the diagram, on the actual board I took 12v from the diagnostics plug and regulated it down to 5v.

max232.jpg

OBD2 - specifically that used in Jap cars is 12v High 0v Low logic:

ISO 9141-2 (Asynchronous serial communication at 10.4 Kbaud)

Pin 7: K-line bidirectional for communication

Pin 15: L-line (optional) unidirectional for waking up the ECU

Idle signal levels are high

Signals are active pull-down to 0V (0.00 to 2.40)

High signal voltage level: +12V (min/max 9.60 to 13.5)

Up to 12 message bytes, excluding frame delimiters

Bit Timing;

UART signaling at 10.4K baud, 8 data bits, no parity, 1 stop

By the looks of the ELM datasheet, the chip is just a PIC micro in disguise.. you need to add most of the external circuitry (to get it to 12v for the OBD and RS232 voltages) yourself..

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 Durty-Sanchez']

SSM is TTL.. here's the first setup that I used (before switching to USB). All that you'd need to do is remove the max232 chip and replace it with the AVR. This setup hooked in to the com port on a PC. I've just assumed a 5v source in the diagram, on the actual board I took 12v from the diagnostics plug and regulated it down to 5v.

Yeah but the ELM chip does all of the INIT stuff that I really cant be bothered coding for. ALSO, if SSM is TTL, then thats perfect but info on it seems to be quite hard to find. ie what pins does it operate on, how to initialise, what commands does it accept (if not the same as obdii pid's)

But thanks for the info so far - its awesome!

[quote name='gazzy2000 said:

Hey chrisjunkie...

I have a TTL-232R (USB to TTL serial) cable here that i was using on my v5/6 ECU.

Log1call was helping me get it connected with evoscan.

I am happy to send it to you to borrow and if you get it connected and want it just give $20

Let me know?

Let you know... b]YES PLEASE![/b] It would be really cool to use to get used to SSM comands etc. Will borrow in beginning but may find that my netbook in a mount actually is a lot cooler than an LCD AVR circuit + it requires less programming :P Are you Auckland based?

ALSO, do you have the software?

EDIT: never mind, seems like they find it hard stopping google getting onto the download link page ::)LINK HERE (not sharing a pirated link merely a search result to a page hosted on THEIR servers)

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 chrisjunkie']

[quote name='Durty-Sanchez said:

Hey chrisjunkie...

I have a TTL-232R (USB to TTL serial) cable here that i was using on my v5/6 ECU.

Log1call was helping me get it connected with evoscan.

I am happy to send it to you to borrow and if you get it connected and want it just give $20

Let me know?

/quote]

Let you know... YES PLEASE! It would be really cool to use to get used to SSM comands etc. Will borrow in beginning but may find that my netbook in a mount actually is a lot cooler than an LCD AVR circuit + it requires less programming :P Are you Auckland based?

ALSO, do you have the software?

EDIT: never mind, seems like they find it hard stopping google getting onto the download link page ::)LINK HERE (not sharing a pirated link merely a search result to a page hosted on THEIR servers)

hey Chris,

PM me a postal address for that cable.

I do have the evoscan software, but its soo cheap you may as well juts buy it ($26 i think?)

I am timaru based, i am the chap that did the Bh5 TT to ST and had Reubens help.

Cheers

Gary

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 chrisjunkie said:

So I got the cable, nicely with a subaru logo on the front (Nice one Garry really made me laugh) ;D

Can't seem to get it to initialise though. Maybe during the single turbo conversion the pins changed??

Thought youd like that! :D

Where are you trying to connect the cable to? ECU itself or OBD plug?

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right, i had a dig through my notes (remembering my car is a 2001, just incase its different on yours)

I found, that the tx & rx terminals on the ODB plug dont exist. They have the wires there (on the OBD) but they dont go to the rx and tx on the V5/6 ECU.

Now that you are running a V5/6 ECU, you have to go by what/how that ECU is expecting to tx or rx data.

Which is on plug 136 pin 3 & 4, now my BH factory ECU plugs didnt have ANY cables on that plug or pins i.e. bare holes where pins should be

SO....... i opened up the ECU and soldered some nice wee wires directly on to the pins, of which are at the top and easy to get solder onto. (see pics following)

Use a low heat soldering iron

Feed wires through that blank plug hole.....you know, the third one that no plug actually plugs into.

Drill wee holes to feed the cables through and i used a dirty old chocolate block connector to terminate them, That cable i sent you just screws/terminates into teh chocky connector as long as you get the tx and rx the right way round.

Use different colour wire as you will need to know what color is rx and tx.

Hope that makes sense, i spend hours tracing wires and learning how to get connected.

Now if that gets you connected, log1call will no doubt guide you the rest of the way.(he is good!!)

Oh, and i dont need to tell to be gental as lamb with the ECU, use static precautions and always have the batt dissconnected when plugging in or unplugging :)

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ANd here is my BH factory plug 136 pins 3 and 4, blank..... no pins!

WHere actual V5/6 plug wud have these wires. You can see they are nice and easy to get at (with the ECU case off) for soldering, as the pins are at the top.

The earth wire that is on the cable i gave you, just earth it on to one of the screws on the ECU case/lid.

m4dk2mrz.jpg

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All I can say is WOW!!!! 8) ;D

I will definitely try this but surely I can find where those TX/RX pins from the OBD plug go to and just re-locate them to the right pins on the ECU?? I'm fairly sure with a tiny screwdriver it is actually possible to get the little pins out and put them in a new hole...

EDIT: what pins are RX/TX??? I did have a look at my manuals but cant find it lol :P

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 chrisjunkie said:

All I can say is WOW!!!! 8) ;D

I will definitely try this but surely I can find where those TX/RX pins from the OBD plug go to and just re-locate them to the right pins on the ECU?? I'm fairly sure with a tiny screwdriver it is actually possible to get the little pins out and put them in a new hole...

EDIT: what pins are RX/TX??? I did have a look at my manuals but cant find it lol :P

yea u cud do that for sure.

I just found it easier to solder at the ECU, coz i dont have a OBD plug like the one u brought.

canny remember wot ones are the tx n rx at the OBD, will lok 2moro 4 ya

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