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2 Devices Running off OBD Port?

Kynetic64

Ready to race!
Location
New York
Hey fellas, tried to find info on this but seemed to find a dead end. I have a scan gauge and pgauge, just wondering if I could run both off the OBD port with a splitter. Thanks!
 

DV52

Drag Racing Champion
Location
Australia
hmm........... more than happy to be proven wrong, but I suspect that the answer is NO (Alas).

The VW wiring diagram for the mk7 shows that there is a single pair of wires from the CAN Gateway (what VW calls "J533 Data bus diagnostic interface") and the ODB connector (what VW calls "U31 Diagnostic connection). So communication between any external device(s) that happens to be plugged into the OBD port must happen over this wire-pair.

My understanding of how comms happens on the CAN bus is that only one device can access the CAN bus at any one time. Access clashes do occur when multiple control modules that are installed in the car at the factory want access simultaneously. However, when this happens, the problem is solved by giving each module a pre-set priority right. This ensures that there is a managed way of dealing with multiple modules wanting CAN access at the same time. I suspect that anything plugged into the OBD port likewise has its own allocated priority right in the pecking-order when it needs access to the CAN bus.

If two or more devices are plugged into the OBD port, there is no way of resolving access confict because each device doesn't know it's priority right vis-à-vis that of the other device - result chaos (I think)!!

Don
 

Kynetic64

Ready to race!
Location
New York
hmm........... more than happy to be proven wrong, but I suspect that the answer is NO (Alas).

The VW wiring diagram for the mk7 shows that there is a single pair of wires from the CAN Gateway (what VW calls "J533 Data bus diagnostic interface") and the ODB connector (what VW calls "U31 Diagnostic connection). So communication between any external device(s) that happens to be plugged into the OBD port must happen over this wire-pair.

My understanding of how comms happens on the CAN bus is that only one device can access the CAN bus at any one time. Access clashes do occur when multiple control modules that are installed in the car at the factory want access simultaneously. However, when this happens, the problem is solved by giving each module a pre-set priority right. This ensures that there is a managed way of dealing with multiple modules wanting CAN access at the same time. I suspect that anything plugged into the OBD port likewise has its own allocated priority right in the pecking-order when it needs access to the CAN bus.

If two or more devices are plugged into the OBD port, there is no way of resolving access confict because each device doesn't know it's priority right vis-à-vis that of the other device - result chaos (I think)!!

Don


Very in depth, thanks for the answer! In the field I work in, we connect multiple air sensors via a "daisy chain" connection where the sensors are able to send and receive data through 1 cat5 cable linked together. Wasn't sure if the OBD may have been set up similar.
 

takemorepills

Ready to race!
Location
USA
You should be able to run more than 1 device on the OBD2 port, if it is CAN bus.

CAN negotiates to every device on a device-by-device basis, so there is no conflict as DV52 assumes.

I have motor swapped MANY cars with CAN bus drivetrains, and I have even added CAN bus to my boat (Smartcraft).

The only way 2 devices would conflict, is if they both had the same ID, which is highly improbable.

Also, if someone makes an OBD2 port splitter, then well of course it is doable.

Oh, and BTW our work trucks do have 2 devices connected to 1 OBD2 port by splitter. One of them is the PTO command interface, and the other is the idle restriction controller. And, on top of those 2, there is a 3rd OBD2 port for a scanner without disconnecting those other 2 devices.
 

DV52

Drag Racing Champion
Location
Australia
You should be able to run more than 1 device on the OBD2 port, if it is CAN bus.

CAN negotiates to every device on a device-by-device basis, so there is no conflict as DV52 assumes.

I have motor swapped MANY cars with CAN bus drivetrains, and I have even added CAN bus to my boat (Smartcraft).

The only way 2 devices would conflict, is if they both had the same ID, which is highly improbable.

Also, if someone makes an OBD2 port splitter, then well of course it is doable.

Oh, and BTW our work trucks do have 2 devices connected to 1 OBD2 port by splitter. One of them is the PTO command interface, and the other is the idle restriction controller. And, on top of those 2, there is a 3rd OBD2 port for a scanner without disconnecting those other 2 devices.

takemorepills: Hmm........ Ok - if you say so!

As I said in my original response -more than happy to be proven wrong.

I wouldn't have thought that the devices that are plugged into the diagnostic CAN bus (as distinct from the "other" CAN bus networks) via the OBD port actually have a specific hex address that can be allocated (unlike the normal control modules which are allocated address numbers as per the CAN Gateway listing).

But if you have done this before on VW vehicles - then that's all the information needed!
Don

Kynetic64: If you do build a splitter - I would be interested to hear of your observations
 

Kynetic64

Ready to race!
Location
New York
takemorepills: Hmm........ Ok - if you say so!

As I said in my original response -more than happy to be proven wrong.

I wouldn't have thought that the devices that are plugged into the diagnostic CAN bus (as distinct from the "other" CAN bus networks) via the OBD port actually have a specific hex address that can be allocated (unlike the normal control modules which are allocated address numbers as per the CAN Gateway listing).

But if you have done this before on VW vehicles - then that's all the information needed!
Don

Kynetic64: If you do build a splitter - I would be interested to hear of your observations


a friend of mine gave me a splitter he bought a few months ago. I will try this today and get back to you.

Thanks for the info fellas!
 

takemorepills

Ready to race!
Location
USA
takemorepills: Hmm........ Ok - if you say so!

As I said in my original response -more than happy to be proven wrong.

I wouldn't have thought that the devices that are plugged into the diagnostic CAN bus (as distinct from the "other" CAN bus networks) via the OBD port actually have a specific hex address that can be allocated (unlike the normal control modules which are allocated address numbers as per the CAN Gateway listing).

But if you have done this before on VW vehicles - then that's all the information needed!
Don

Kynetic64: If you do build a splitter - I would be interested to hear of your observations

DV52, it should be OK, to be clear I did not ever do it on a VW, but the CAN protocol is the same for all devices that use it. The CAN comm circuit in the OBD2 port is the same 2 wire circuit running throughout most of the car. Whatever the device ID is, it is highly unlikely that it will be the same as any other devices, whether that be scanners, diagnostic tools, power window modules, etc.

And yes, anything plugged into an OBD2 port will have a device ID if it wants to interact with CAN data. I do not know if every scanner on the market has the same device ID or if they all have a different ID, I don't know that much about it.
 

hal

Ready to race!
Location
Bahrain
If it would help I connected two head units (stock infotainment and an aftermarket android) to the same port and canbus socket and they both worked without issues. They both listen to all canbus traffic and they both respond.

They do fight over MFD sometimes (because both want to display phone/audio information) but the point is both work by tapping into the same exact can+ and can- wires in the quadlock harness.
 
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