Moe RFD Harnesses: RFD Telem Tower Schematic RFD to PIXHAWK Harness
Note the RFD900 is different from the RFD900X
Power/Thermal Analysis
From datasheet https://files.rfdesign.com.au/Files/documents/RFD900%20DataSheet.pdf power output for RF is 30 dbm max is roughly 1 Watt. Max input power is 5.5 V at 1 A roughly gives 5.5 W. This means worst case the RFD device is dissipating 5.5 - 1 = 4.5 W of power as heat. at 85 C max you could do the math for thermal impedance of case and ambient but it becomes kinda approximate.
Testing the communication between 2 RFD900X.
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Hal library reference web: https://www.disca.upv.es/aperles/arm_cortex_m3/llibre/st/STM32F439xx_User_Manual/group__uart__exported__functions__group2.html
A flight harness for the RFD900X can be found in RFD to PIXHAWK Harness