Obdii Vpw

[DynamoBen]

I recently came across an article published in Circuit Cellar last year that describes a automobile Fuel Consumption Gauge. The devices ties into the OBDII system and gathers data and then actuates the gauge to display fuel consumption.

www.circuitcellar.com/advertise/ cc-advertising/Lightner-183.pdf

After reading the article I thought it might be fun to do this with a pic and PICBasic. After a little searching I found the following website which nails down the hardware side but I'm a little lost on software. (I don't read C so I am unable to port the code to PICBasic)

http://home.pacific.net.au/~ijhope/pcm/vpw_to_rs232.htm

I would assume I could read the VPW strings with Pulsin, and write with Pulsout but I'm not certain about that.

NOTE: I know about the ELM Electronics ELM322. (I own one) I'm looking to directly interface with the buss.

Thoughts, help, suggestions.

[schu4647]

I have never done this, as I have a NeoVi, but this site looks promising. These IC's look like they do the entire conversion for you. Keep in mind every automaker does it a little different and you need to know how to intperpret the data. Here is the site:

http://www.elmelectronics.com/index.html

Looks like they have a chip for every variation of the protocol.

[schu4647]

Actually looking at those, they are just PIC's that somone programmed and is selling. I thought they were VPW specific IC's.

[DynamoBen]

If you take at look at my initial post:

"NOTE: I know about the ELM Electronics ELM322. (I own one) I'm looking to directly interface with the buss."

To be honest I not only have the ELM322, but I have all five of the elm devices. I was a beta tester for the most recent release. I'm interested in tying my pic to the data bus like the article did.

I own, have read, and understand the SAE documents pertaining to the communications protocol. The commands for communication are standard. (J1850) The thing at differs between manufacturers is at the physical level vpw, pwm, iso, and can. I'm just not sure how to implement vpw in PICBasic and would like to.

[schu4647]

I aplogize, I did research between reading your post and posting a reply. I would do it the exact same way you are proposing. It sounds like an interesting project. I would think it would work as long as you have time to send the message out on RS232 between bus messages. I don't have the protocol in front of me, and it has been a long time. I wouldn't think it would be that difficult. I would think it would really only take 4 tests. If the bus is low for long, low for short, high for long, high for short. Throw it all in a for next loop to capture the whole message. I would only be worried about missing part of the next pulse width while you are testing the previous one. It would defanitely work though if you get that aspect of the the timing down, and you don't get backed up by sending the RS232 out. I saw some VPW tranceivers out there, that seemed to make the send and receive side a little simpler. It has been a long time since I did VPW, I am more into CAN now. Hope it works out for you.

[DynamoBen]

The intention is not to do a VPW-RS232 convert, that is why I who the ELM chips. I am just interested in getting a pic onto the buss and communicating.

[schu4647]

I think this might help a lot. I am looking through a J1850 book and it shows a few examples. They use a HIP7010 or HIP7030A2 as the handler. This should take care of the protocol aspect. Hook it into a HIP7020 Transceiver and you should be good to go. I didn't study the data sheet too much, but I think this will simplify things. Here is a link to the data sheet for one of the handlers.

http://www.intersil.com/data/fn/fn3644.pdf

[DynamoBen]

One Problem:

http://www.intersil.com/support/ps.a...r=HIP7010&R1=B

HIP7010B=Retired
HIP7010B96=Retired
HIP7010P=Retired


Discontinued, this has been the case for all the ICs that peform this type of function. I even checked Motorola/Freescale, they used to make chips similar to the above. However they have moved these functions onto their microcontrollers and discontinued the outboard chips.

[schu4647]

That wipes me out of ideas. I guess PIC's have limitations.

[DynamoBen]

More detail on VPW:

http://www.intel.com/design/intarch/papers/j1850_wp.htm

[DynamoBen]

After looking this over I think the inbound vpw could be read by either looking at the pin state and comparing it with Timer1 or using the CCP module and having this occur in the background. I have determined that Pulsin would be too slow to grab the whole packet.

I'm kind of leaning toward the CCP module after reading the following.
http://ww1.microchip.com/downloads/e...Doc/41215B.pdf
(see tip #3 and #4)

The problem is I have never used the CCP module before. Anyone have any experience with it? Advantages/Disadvantages to CCP versus Pin State/Timer1?
Examples might be helpful.