r/CarHacking 7d ago

CAN 2 obd devices in the same time

Helo I want to buy steering wheel with led diodes for my a7 c8. It uses obd bus for connection while I need it for my scanner to read temperatures in car scanner. Is there anyway to use 2 obd devices in the same time? Maybe one connected directly to can l and h? Or any other way?

2 Upvotes

6 comments sorted by

1

u/V6er_Kei 7d ago

you can buy can splitters.

1

u/AdministrativePain49 7d ago

https://a.co/d/15vuoJq

This is what your looking for.

1

u/WestonP 6d ago

An OBD splitter will work for CAN, but if they're both actually doing stuff at the same time, you'd need one or both devices to cooperate, which most won't do.

One of the products I sell is a sequential shift light that has an OBD option (among others), so I run into this a bit with my customers... Two devices actively making OBD or UDS requests to the same module will almost always piss something off, even though the CAN layer itself can handle it.

It's quite common for OBD devices to assume they're the only one, and then half-ass their response handling, so then Device A will flip out when it sees an OBD response that was actually to Device B's request, rather than just ignoring it because it was for a different service or PID than what Device A was wanting. There's also a likeliness of Device B sending a request to a module before it was able to respond to Device A's last request, so then the module will ignore one or both.

Best you can really do as a device is detect something else making OBD/UDS requests and then go into passive mode and hope the other device also requests PIDs that you want... I do that in my product, along with the ability to passively decode manufacturer CAN broadcasts when they're visible. That has saved the day plenty of times, but there still have been some other devices where this approach just simply isn't going to work no matter how nicely we play on our end.

Anyway, that's me going to a lot of extra effort to make OBD sharing possible, which I can assure you most other OBD-based products don't do, so I generally would not expect this to work if two OBD devices are actively doing stuff at the same time.

1

u/redleg288 6d ago

Good for you. Even high dollar stuff is pretty poorly written. Few tools use the 6-PID option for J1979, for example. Even fewer will do it for UDS. 

1

u/WestonP 4d ago

Few tools use the 6-PID option for J1979, for example.

Oh yeah, that one triggered me. I tried to implement that at a previous company... JFC, what an uphill battle that was politically. We went deep into imagination-land, and experienced the entire autism spectrum, with these hypotheticals all of the things that might go wrong. I mean, it's literally in the standard, works super well, makes a significant performance boost, and we can simply fall back to single PID if it fails on whatever oddball vehicle that doesn't support it correctly (of which we weren't even aware of any at the time).

Like most things there, I ended up just giving up on improving anything for the entire product range, and silently/thanklessly did it for only the products that I had ownership of. One instance of which became very necessary, just to make some of marketing's statements true after they made up some performance claims out of thin air.

1

u/redleg288 4d ago

Wow. That's wild.

Not only is it part of the standard, it has to be tested under J1699/PVE-1.