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Help needed on Powerline TRx ST7540

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

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  1. blisca

    blisca Guest

    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

  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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