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SDI-12 Bus Interface

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This simple circuit interfaces a 3.3V microcontroller with a 1200 baud UART to the SDI-12 bus.  Q1 is a voltage translator which translates the 5V signal from the SDI bus to 3.3V.  The tristate buffer, also translates the 3.3V from the UART TX line to 5V, and allows the microcontroller to place the line in the high impedance state when not transmitting, but deasserting the DIR_OUT line.  

 
SDI-12 Bus Interface Schematic

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