USB to TTL adapter needs to be connected following the "null-modem" schematic. So
TX (adapter) -> RX (Teensy)
RX (adapter) -> TX (Teensy)
In addition I suggest you test the connection accessing Loop Controller serial port using a simple Terminal program like
Termite. Be sure the Teensy serial port has the proper setup like 19200,N,8,1 and then send some commands to see if and how the controller responds.
73' Enzo
iw7dmh