Electronics > Projects, Designs, and Technical Stuff
Automotive Diagnostic - 24v to 12v Converter for K-Line, L-Line, J1708, J1939
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symp:
I have a feeling I may be on my own with this haha.

Dave if you see this....You really need to cover this subject as it links to right to repair highly. John Deere are standing in court with Apple on this subject. John Deere is trying to make my job difficult as Apple makes Louis Rossman's job difficult.

Australia has only just passed a law requiring car OEM's to provide independent mechanics with service data. About 10 years behind Europe and USA. Let alone trucks, which Europe and USA has covered for years. And then we have industrial, agricultural, construction and earthmoving equipment. Full of electronics with absolutely no basic diagrams available to independant mechanics in Australia, let alone actual schematics to repair modules. A John Deere John Deere 2018 7R tractor has up to 33 computers all communicating via 2 seperate CANBUS networks.
Rerouter:
Ok, so to hit the 2 most common plugs to convert for

Ground, J1708 and J1939 are passthrough, Battery is stepped down to a lower voltage, up to 3A is the specificiation from memory, but most tools barely pull more than an amp,

K and L Line will need either an active converter, e.g. 2 K-line trancievers back to back with RX and TX tied for the easiest implementation for a few dollars, or something more crafty if you want to DIY a bidirectional converter for it, remembering low is not ground, but less than 1/3 Vbat, equally high is above 2/3 Vbat, and this is ratiometric, a flat car at 10.8V and a running car at 14.2V do have different threshold levels.

https://pinoutguide.com/CarElectronics/sae_j1708_pinout.shtml

symp:

--- Quote from: Rerouter on August 08, 2019, 08:18:25 am ---Ok, so to hit the 2 most common plugs to convert for

Ground, J1708 and J1939 are passthrough, Battery is stepped down to a lower voltage, up to 3A is the specificiation from memory, but most tools barely pull more than an amp,

K and L Line will need either an active converter, e.g. 2 K-line trancievers back to back with RX and TX tied for the easiest implementation for a few dollars, or something more crafty if you want to DIY a bidirectional converter for it, remembering low is not ground, but less than 1/3 Vbat, equally high is above 2/3 Vbat, and this is ratiometric, a flat car at 10.8V and a running car at 14.2V do have different threshold levels.

https://pinoutguide.com/CarElectronics/sae_j1708_pinout.shtml



--- End quote ---

That is what my circuit takes care of on the K-Line, based on "Bi-directional level shifter for I²C-bus and other systems. AN97055"  by Philips.


Also OBD2 is only used on the diagnostic connector side in this case. Here is an example of some of the standard industrial connectors that I deal with. That is just some standard ones, I have an adapter case with over 30 adapters for different machinery, sometimes it is just a case of using wire probes and the right software through the right adapter using the right protocol.  :-DD
Rerouter:
I will warn again, it is not a simple level shifter, you will need to bias things so they only respond at the right thresholds, there is a stupid amount of Kline devices that have about a 3.6V low signal on 12Vbat and do not ever come close to ground, I've been bitten by this myself when I made my own simulator interface for a few bits of hardware. the gap is important because any alternator spikes get coupled in to these signals, and have seen ripple on these signals as high as 300mV.
symp:

--- Quote from: Rerouter on August 08, 2019, 10:47:12 am ---I will warn again, it is not a simple level shifter, you will need to bias things so they only respond at the right thresholds, there is a stupid amount of Kline devices that have about a 3.6V low signal on 12Vbat and do not ever come close to ground, I've been bitten by this myself when I made my own simulator interface for a few bits of hardware. the gap is important because any alternator spikes get coupled in to these signals, and have seen ripple on these signals as high as 300mV.

--- End quote ---

The diagnostic adapter is designed to work within the limits of each OEM, it just wont tolerate anything over 18v. So I am trying to keep it simple and let the diagnostic adapter do the work and this converter simply stop anything over 18v being passed through. The other design I have is a couple of npn bjt's back to back.

The mosfet on the K-line is not referenced to ground in my schematic, the diagnostic adapter does that depending on the OEM it is talking to, Cummins, Detroit, John Deere etc. As far as I can tell, if the ripple is less than the gate threshold voltage it shouldn't cause any problems? I am probably completely wrong  :-DD

Do you think the NXP 33660 or Infineon TLE7258D would work well as back to back transcievers?
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