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We have an aftermarket cruise control installed (in Suzuki Swift), which is connected to the OBD-II connector to access the CAN bus (according to the installation manual, it uses just CAN-L and CAN-H pins from the connector).

Now I have bought an OBD-II Bluetooth module. Could it be used at the same time as the cruise control?

If I understand the Wikipedia page on CAN bus properly, devices are connected to the bus parallel and terminator resistors are only at the two ends of the bus line. So it should be possible to have both devices connected to the OBDII connector... About using the devices at the same time, I think that would depend on which ID the cruise control is assigned to (so that it would always get priority above the Bluetooth module).

To summarize the questions:

1) Is it possible (and safe) to have both these devices connected to the OBD-II connector at the same time?

2) Is it possible to use them at the same time? Is it common for the diagnostic devices (like the ELM327) to have the lowest ID on the CAN bus to not interfere with other devices on the bus?

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    \$\begingroup\$ I'd test it out in an empty parking lot first. \$\endgroup\$ – Joel B Jul 20 '12 at 17:35
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I think this will be safe. My reasoning is this: - the vendor of the cruise control hopefully made sure not to interfere with the OBD-IDs (AFAIK 0x7DF and 0x7E8, both fairly high ids for an 11-bit identifier). - the bluetooth module will (hopefully) only use OBD-ids.

If you don't trust the vendors of both devices, you should not connect them to your vehicle even only one at a time. CAN is a very robust protocol, the worst case scenario would be for the bus flooded with collisions for a short period of time until one of the devices involved would stop sending messages. I'm not sure what exactly would happen in this case, but I'm positive that uncontrollable acceleration will not happen: Manufacturers are aware that a failure of the CAN bus can happen any time (just think of a short between both lines).

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