SDI-12 Bus Interface
SDI-12 is a arcane protocol used in environmental sensing. It consists of 2 wires: data and ground and runs at 1200 baud. The bus is bidirectional, and the logger is the master, and the sensors are slave devices on the bus.
It's not a very easy protocol to implement. Fortunately, you can purchase translators from Vegetronix which all easy implementation of the SDI-12 protocol.
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 de-asserting the DIR_OUT line.
SDI-12 Bus Interface Schematic