The Tandem dual-band receivers are unlike any other FrSky 2.4Ghz or 900Mhz receivers, they work simultaneously at both 900Mhz and 2.4Ghz frequencies. That means Tandem receivers provide not only the low latency signal and long range control, also benefits from enhanced level of high reliability and anti-interference performance.
The Tandem series receivers adopt the triple antenna design that provides multi-directional wide coverage for the remote signal. The 900Mhz antenna is a newly designed small form factor, but has the equivalent high-performance as with previous 900mhz antennas.
TD MX offers 4 PWM channels (no pins) on a small and lightweight board, and it also has multiple ports onboard like SBUS Out, S.Port and FBUS. With the FBUS protocol, the Tandem series receivers can open up the possibility of seamlessly pairing with multiple telemetry devices (Neuron ESC, Advance Sensors, etc.) as well as simplifying the builds setup. In addition, the data (Power & Signal related) under unusual status during the flight can be recorded by Tandem receivers through the built-in black-box function.
Manual / firmware page here