In a KNX TP power supply, the choke presents a low impedance to DC and a high impedance to AC.

The choke is an inductor (typically several mH) wired in series between the DC source and the bus line. By the nature of an inductor, its impedance is essentially zero for direct current and rises with frequency for alternating signals. The DC bus voltage (around 29 V) therefore passes through unimpeded to power the devices, while the AC components of the bus telegrams (the 9600 bit/s modulated signal, with a bandwidth around 10 kHz) see a high impedance and are not short-circuited by the supply. Without the choke, the supply's low DC source impedance would swallow every telegram.

Never replace a KNX power supply with a generic 30 V DC industrial supply — the missing choke kills bus communication immediately, even though the bus voltage looks correct on a multimeter.