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Help needed on Powerline TRx ST7540

Discussion in 'Electronic Design' started by blisca, Oct 10, 2012.

  1. blisca

    blisca Guest

    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
     
  2. blisca

    blisca Guest

    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
     
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