PCB, ASSY, STX INVERTER I/O BOARD I/O Inverter Board.  Input/Output Inverter board.   Allows the transmitter Inputs to be controlled by positive logic.  Needed with Gen II if you want to invert logic.  Not needed in STXe units.

STX-LP Gen 2 GPIO requires a negative logic / signal to ground for remote control.  To use this transmitter with a positive logic / +5V signal system, such as the FA-2 Main Alternate Controller,  the +5v signal must be converted to GND.  

The 919-4001 I/O Inverter Board accomplished that task.