The X-band downlink path is shown by the blue line running from Transponder 2, to the TWTA Interface Unit and Travelling Wave Tube Amplifier 2 (TWTA2), eventually to the High Gain Antenna (HGA). This is the current spacecraft configuration.
A critical role of the TWTA interface is to automatically maintain the downlink in the event of a TWTA2 failure. If the spacecraft senses a malfunction of the active TWTA, it will shut the suspect unit down and start an automatic power-on sequence for the redundant TWTA. In such as case, the TWTA Interface unit will reposition the waveguide switch to connect the new TWTA to the High Gain antenna.