Help needed on Powerline TRx ST7540

[blisca]

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
http://www.st.com/internet/com/TECH...AL_LITERATURE/APPLICATION_NOTE/CD00143379.pdf
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

 

[blisca]

Hi,
please i ask some help or hints about ST7540.

simply ,the IC was damaged,likely by previous short circuits

i have postponed it,due to the weakness of my homemade PCB

Now output signal is about 1.8 V wide on 600 ohms

Thanks for attention