Electronics > Projects, Designs, and Technical Stuff
Automotive Diagnostic - 24v to 12v Converter for K-Line, L-Line, J1708, J1939
symp:
--- Quote from: carl0s on August 07, 2019, 09:29:09 pm ---The K-Line itself can already be up to 36v, certainly for the L9637D chip anyway.
e.g. https://www.ebay.co.uk/itm/223221235158
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Hence my design, it should reference to the system voltage, be it 12v or 24v or over. In my experience I have not encountered industrial engines running 36V.
symp:
--- Quote from: carl0s on August 07, 2019, 09:31:24 pm ---I suppose that only helps if you want to make your own interface though :-/
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I am only translating voltage levels to suit a 12V diagnostic interface.
symp:
--- Quote from: MosherIV on August 07, 2019, 09:47:43 pm ---ODB2 is a CAN based protocol.
There is a large amount of data integrity checking going on in hardware. Look at the bit stuffing section from wikipedia
https://en.m.wikipedia.org/wiki/CAN_bus
I do not think that J1708 is compatible with CAN. The kvaser site indicated that the basic data transmission part only uses a single checksum byte as the last byte of the 6 bytes - totally different from CAN.
--- Quote ---Most people are completely unaware most new cars use at least a 250Kbit/s bus, the usual reaction is "what the hell they do not need such speed"....CAN FD uses 1Mbit or more :rant:
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The number of inter-connected sub-system in vehicles now mean the even CAN-FD may not be enough!
I have spoken with colleagues that think automotive ethernet will become the new defacto standard.
One of my friends assumed that the ODB2 CAN interface would allow him to access the vehicle CAN bus.
:-DD
Most of the vehicle sub-systems are on a private hidden CAN network. You actually have to tap into it from the vehicle loom somewhere.
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Aussie response.....Yeah, nah, obd2 is the physical interface
symp:
--- Quote from: Rerouter on August 07, 2019, 09:52:14 pm ---J1708 is a RS485 tranciever, a lot of american trucks still use it, and volvo buses up to about 6 years ago still had them in combination with J1939, each having different data types. you can decode it as serial data on a computer fairly easy, it was a low baud rate.
J1708 is nice and slow, J1939 at 500kbit is very common these days with Mercedes, and a few big manufacturers are about to make the jump as they are consolidating there sub brand components. 1mbit is still probably about 3 years off being using in a common truck / bus, if someone does do it, I would expect Mercedes would lead that. (Current year sprinter van has 1mbit canFD as far as my scope probing could tell, as nothing I had could talk to it)
K line has special transceivers as well, and I would strongly recommend using one, there is a stupid amount of devices out there that do not pull to ground, but only about 1/3VCC, 10400 baud is the most common, the 5 baud is only for the handshake.
If the diagnostic tool is built using these transceivers then it should cope fine for J1708/J1939 with a reduced battery feed and the other optional IO pins disconnected. K/L-line would involve looking at what chip they used, or doing a double level shifter if you can't open it, as the signal is a ratio of the battery voltage,
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Best reply with knowledge so far.....Forget software and IO, I am simply looking to translate industrial engine logic levels to fit within the spec of a 12V diagnostic device....Software is already made and tested by a company who makes diagnostic equipment called Launch. I am basically trying to replicate the 24V industrial/truck converter that they sell for $1200AUD, as the OEM vehicles they support are all done in software via an STM32. Once I have one in hand I will be able to look at the pull up and down resistors they use.
symp:
--- Quote from: carl0s on August 07, 2019, 11:15:19 pm ---
--- Quote from: MosherIV on August 07, 2019, 09:47:43 pm ---ODB2 is a CAN based protocol.
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In some cases OBD2 is carried over CAN, and in many other cases it isn't. So it's not CAN based at all really.
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CAN is a protocol, OBD2 is a physical plug.....It is like saying sometimes TCP is carried over CAT5.
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