I have the TDA5051A on a 12VDC power line using the schematic below. 7.372MHz independant oscillator, 115.2kHz carrier frequency. I've tried both 600 and 1200 baud communication.
I can get the transmission to work over short distances (<4ft), but once it gets to about 6-8ft, the communication starts to get lost. See the attached oscilloscope plot showing the degradation over about 4 ft of cable. CH2 is on the output of the transmitter and CH1 is at the receiver at the other end of the cable. Seems like the carrier signal is getting washed out a bit over the distance of the cable and I'm not sure why that is happening.
Is the amplitude of the carrier signal expected to be so small (~100mV)? I've done simulations of the coupling circuit and it should be much higher (~1.5V).