In some rare cases you might experience problems when connecting the V5 USB interface to the IBUS without having a PC attached.

When the PC driver recognizes the interface once you're set.

The problem results from a faulty IBUS protection circuit of the Melexis chip:

Here's how it (should) work:
There's a protection circuit in the TH3122 to prevent the IBUS from being pulled low for an extended period of time. If the IBUS is pulled low for an extended period of time no device can communicate on the bus anymore.

Unfortunately this circuit has one problem: when the transmission line (that comes from the USB->Serial adapter) is low at power-on,
the circuit does not work, and the IBUS will be stay low as well.
Nothing on the IBUS works anymore....

So when you connect the power to the interface without the USB connection (with a running PC) then you end up with exact that scenario.

When the Transmission line gets high once, the protection circuit works fine from then on, even if the transmission line will stay low forever afterwards (when the PC shuts down and the USB->Serial adapter get not power anymore).

This working condition lasts as long as the Melexis chip has 12V power.
If it looses power the faulty protection circuit is reset and will pull the IBUS low until the USB side gets power and is initialized by the driver.

So the solution is to use a permanent +12V power supply for the V5 interface.