It's been a few weeks since I've read the UART sections in the datasheets . . . but from RS485 half duplex discussions, and a vague memory of the UART being capable of inverted operation, it would seem the Teensy3 should be capable of interfacing to the 1200 baud 7E1 inverted half duplex SDI-12 protocol with nothing more than a 3/5V level shifter.

Has anyone played with SDI-12 sensors and the Teensy3?

Alistair