TH3122 KBus transceiver seems blocking its own TX
You should force SEN/STA to 0 in order to transmit, as the datasheet clearly indicates.

Also, see the "Constant Low Switch Off" description. It means that your TX line should not be held low for more than 3 ms. If your protocol needs to transmit "0" for longer than that, you have to insert short "1" pulses periodically to prevent the transmission from being disabled.

UPDATE

I am looking at the documentation, and I have no idea how this wiring can work at all. If you have any links to the K-Bus specification it would be helpful.

The problem is, the output driver is a PNP open emitter. So, without pull-up all you can hope to get is a short spike on the input capacitor, exactly what you see on your scope. At least now you are getting those spikes on each TX pulse, and there is not any "blocking" as in the title.

From some obscure references there should be +12&nbsp;V on the bus in idle state. Do you have that? I guess if you were to connect to a real BMW bus it would be pulled up somewhere by the car. If you do the experiments on your desk then you have to do it yourself. I don't believe the leakage current from the transistor base to emitter could possibly drive the entire bus up, unless you have a dozen devices connected to the bus simultaneously.

Here is the circuit according to documentation, including driver stage of TH3122:



UPDATE 2

Here is tiny bit of information found here:
"network load: power-up resistors in Master and Power-up modules". This confirms my guesses above - you need to either test your adapter while connected to actual bus, or you need a 1&nbsp;kΩ pull-up resistor on a bus to test standalone chip operation.

UPDATE 3

The internal 5V regulator not only outputs power for MCU, but also supplies chip logic and I/O circuits. So, it has to be enabled for proper operation. It requires all the capacitors as per "Figure 8 - Application Circuit" in the datasheet.

The EN pin can be permanently connected to VS or it can be controlled by MCU after MCU has been woken up by bus traffic.