Multiple devices connected to OBD port?

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DV52

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Simple question (and I probably should know the answer, but I don't) - can a device like a VCDS dongle be connected to an OBD port at the same time as another device (OBD gauge, say) on a VAG vehicle? Would clearly need a "Y" splitter cable but would the CAN Gateway happily cohabitate with both devices?

Don

PS: If the answer is yes, please explain how comms conflicts are resolved
 
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Gavra

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If you want to read data everything should be fine but I would not try to re-code modules with Y adapter. CAN gateway should not even recognise both devices because messages on CAN are transmitted always even when u r driving without scan-tool connected.
We had some CAN decoders connected on the back of DLC and people were driving for 6 months together with servicing and no issues reported.
 
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Uwe

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Simple question (and I probably should know the answer, but I don't) - can a device like a VCDS dongle be connected to an OBD port at the same time as another device (OBD gauge, say) on a VAG vehicle? Would clearly need a "Y" splitter cable but would the CAN Gateway happily cohabitate with both devices?
I don't think a definitive answer is possible. Simply having two devices physically connected generally isn't problematical, but problems may arise depending on what the two devices are doing.

-Uwe-
 
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DV52

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Uwe/Gavra: many thanks for your replies. Guess I'm still confused as to how the Diagnostic CAN bus is able to resolve comms conflicts - I didn't think that J533 was smart enough to dynamically allocate address IDs to multiple connected devices.

But only one way to find out - time to heat-up the soldering iron and make a "Y" splitter!!

Don
 
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Gavra

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Make sure you make parallel NOT serial Y adapter ;)
 
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smokey08

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My experience from using OBD trackers (with built in remote OBD2 readers) on Y splitter cables is that you will have trouble communicating with the same module.
I run these on a fleet of mercedes sprinters and have to unplug them if I need to connect to the engine ECU. All other modules I can connect to fine.
 
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DV52

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Uwe/Gavra: many thanks for your replies. Guess I'm still confused as to how the Diagnostic CAN bus is able to resolve comms conflicts - I didn't think that J533 was smart enough to dynamically allocate address IDs to multiple connected devices.

But only one way to find out - time to heat-up the soldering iron and make a "Y" splitter!!

Don

OK - I finally got around to trying this: Got a "Y connector and hooked up my trusty HEX-NET and another manufacturer's OBD diagnostic dongle (whose identity will remain nameless) to my test bench. I still don't know how it's possible, but it appears that both diagnostic devices are very happy operating with the arrangement.

I can separately navigate through the modules with either device (almost simultaneously). Everything seems to work OK - albeit, I did try to access the same adaptation channel with both devices and not surprisingly I got error messages when the previously stored value of one of the units no longer applied (because it was changed by the other unit after the first unit polled the control module).

Can someone explain how this is possible? Does HEX-NET have a secret hex address that it sends to J533 -to deal with comms conflicts?

Don
 
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Gavra

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You can't have communication conflicts with reading(decoding) data from any ECU. Only situation when you can have problems is coding/programming when data is sent in bi-directional mode and can cause conflicts.
 
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PetrolDave

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The CAN bus protocol allows multiple masters all active at the same time, determining which master has control of the bus for any particular message is done by every master doing a bit by bit comparison of the data on the bus with the data it is trying to send and as soon as there is a mismatch then that master becomes passive for the remainder of that message.
 
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Uwe

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You can't have communication conflicts with reading(decoding) data from any ECU.
I'm sorry, but I disagree. Given the way the CAN protocols work, it is entirely possible for two "testers" that are both requesting data to conflate the responses from the car. Does that mean it's likely to happen? Not necessarily. But I don't accept that it "can't". :)

-uwe-
 
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Sebastian

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...in fact ODIS does do requests to multiple control modules at once under certain conditions.
 
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Uwe

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...in fact ODIS does do requests to multiple control modules at once under certain conditions.
So does VCDS, under certain conditions. But that is not at all the same as having multiple devices connected to the same car at the same time, particularly when device A doesn't know what device B is doing and vice/versa.

-Uwe-
 
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DV52

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^^^^ Gentle-people: many thanks for your contributions. Clearly there is much I need to learn about this interesting aspect of OBD - guess that some further research is appropriate. My observations just seem to be incongruous with what I have understood thus far from colleagues here and from my musings after reading VWs material. I'm not about to try modifying ECU values with this set-up on a live vehicle- it's just an academic exercise to test the envelope of my scant knowledge, which obviously is sadly lacking! But it's always fun learning!!

Don
 
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Gavra

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I'm sorry, but I disagree. Given the way the CAN protocols work, it is entirely possible for two "testers" that are both requesting data to conflate the responses from the car. Does that mean it's likely to happen? Not necessarily. But I don't accept that it "can't". :)

-uwe-
Read as shouldn't :)
 
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