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Fully Isolated RS-485 bus - LTM2881-3

Question asked by Louis2 on May 9, 2018
Latest reply on May 16, 2018 by Louis2

Hi,

 

I am interested in the LTM2881-3 isolated RS-485 transceiver.  The typical application circuit on page 24 of the datasheet (also attached) shows the isolated RS-485 bus being completely isolated on all nodes.  What keeps the isolated part of the bus from floating beyond the maximum 560V working voltage of the device due to coupling on long distance cables?  Is it the 1Gohm input to output resistance for example?

 

What use is connecting the screen to the fully isolated 0V?

 

If this configuration works it is of interest to my application as I have floating 12V d.c low voltage supplies with no earth connection on any nodes of the bus.  Any advice on additional surge protection devices and connectivity is also welcome:  

Surges EN 61000-4-5 due to wires >10m.  Open circuit voltage waveform Tr/Th of 1.2/50us, short circuit current waveform of 8/20us:

    • Line to ground voltage ±1kV
    • Line to line voltage ±0.5kV

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