Help needed on Powerline TRx ST7540

Hi, please i ask some help or hints about ST7540. My purpose is to transmit over 24VDC,common GND between ends of the line

i'm certainly doing some error but i dont know what

Symptom : The output stage and the current limit are switching between their limits,in sync with the TX/RX signal,this happens even if the output stage is input is left open,or tied to GND,and with output open ,not driving any line ;i can see it swinging from 0 to 12V,and the current limit pin switches from 0 to 5V(hi level when TX/RX pulse is low). The only way to avoid current limiting it was disconneting 12V supply for the power stage(VCC)

The situation is the following I made a test-board with ST7540 powered by a12V DC power supply,a linear regulator feeds with 5V VDD and VDC. I copied the values of passive components from

The ST7540 has no register written by me,it is in its default configuration. A uC sends a single 2400bps char on TXD pin every 100ms,the lenght of the char is 10 ms less the TX/TRX negative pulse,i use the TX/RX pulse itself to reset the watchdog,enabled by default

I can see signal on the TXout,about 40mV Vpp ,superimposed to the low level of a 0 -5V pulse(low when TX/RX is low) , much more if i disconnect the supply of power stage.

Any hint or link will be appreciated

Thanks for your attention

Diego