TX and RX on single wire
it will be in idle state i.e. 0V
So your receiving device is still driving the output with low impedance to 0V? You will have to switch the pin to a high impedance state in the receiving phase, otherwise the two drivers override each other. This explains why your signal amplitude is reduced to half of the expected amplitude (because you have a voltage divider of the two driver impedances: +12V -&gt; R -&gt; 6V -&gt; R -&gt; 0V).
Be careful with this setup, these settings will result in a high current from these pins. The TX driver might not be designed to withstand these currents for long. But we have no way of knowing this, since you did not tell us what hardware you use...