Strange grounding issue

[NeX]

Yes, a diode between the Arduino and the remote sensor, cathode to Arduino, will protect against the Arduino driving high and the sensor driving low at the same time.

If you want to connect them directly, then use this scheme. Configure the pin as an input, and digitalWrite(LOW) to it when not sending a command. When you want to send a command, instead of doing digitalWrite(LOW), do pinMode(OUTPUT); and instead of doing digitalWrite(HIGH) do pinMode(INPUT).

thanks for the help, but actually i am using direct PORT write to toggle the pin, but i think a diode will be fine,

[dc42]

thanks for the help, but actually i am using direct PORT write to toggle the pin, but i think a diode will be fine,

Then you can use a direct write to the port's data direction register to achieve the same effect.