eBUS Adapter Shield C6 lets my Vaillant control system reboot

[ckxx-github]

I have purchased an eBUS Adapter Shield C6, flashed it with the latest firmware (2025-06-15) and would like to connect it to the eBUS of my Vaillant heating system (aroTHERM plus VWL 75/6, uniTOWER plus VIH).

As soon as I connect the eBUS Adapter Shield to the eBUS, the display of my VRC 720/3 room controller, which is also connected to the eBUS, flickers and then the room controller reboots.

Is this normal behavior? I thought the Shield C6 has no influence on the eBUS? I almost had a heart attack.

I also always get “eBUS signal: no signal”. The ebusd connection is established, the green LED of the shield flickers, but there is always “no signal”.

Can anyone help? I don't know how to further analyze / fix this problem.

[DaveL75]

I am NOT authoritative on this (about to do the same thing myself!) ... have you tried swapping the wires over? I know ebus is not modbus but perhaps there's a comms polarity issue?

[ckxx-github]

Yes, I also swapped the two wires, but unfortunately that didn't help. Every time I tried, the room controller rebooted and there was “no signal”.

I use a multimeter to measure approx. 20 volts on the eBUS, and every few seconds a very short drop to approx. 15 volts. Is this typical for an eBUS?

Could it be that the shield is defective? Or do I have a problem with my eBUS?

[tropfenFK]

Have you solved the problem?
Frank

[john30]

I use a multimeter to measure approx. 20 volts on the eBUS, and every few seconds a very short drop to approx. 15 volts. Is this typical for an eBUS?

every few seconds does not sound right for eBUS and also 15V.
please post a photo of the connector you're using on your heating. it should have "BUS" written on it, but better check against the installer manual if it is the right connector.

Could it be that the shield is defective? Or do I have a problem with my eBUS?

since you have a multimeter, you could disconnect the adapter from everything and check resistance between each eBUS pin and GND of the adapter ad well as 5V and 3.3V, just to exclude a shortcut somewhere (which is extremely rare since every adapter is passed through ATE anyway).