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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note the buad rate used for transmit between device is 46000kbits/s. Inside the rfd config tool it is called the air rate.
the first buad rate is the communication between the computer and the rfd900 note 115200 is beyond most of the computer specification.
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Step 2: connect the Rx pin on RFD 900 to the TX pin configured for the UART communication, and the Tx pin on RFD 900 to the Rx pin configured for the UART communication on STM32.
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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