I found that my preliminary setup works OK, but unstable. There are a couple problems, including: (1) FS100A input is a NPN BJT 8080, with base input series I tried HC04 inverter (twice), with Pico GP0 3V3 output and found it more or less OK, but I think

resistor of 4k7, resistance/impedance is too low and therefore loads down input signal (Pico or sig gen PMW signals), (2) therefore a up level shifter is required. I use TBX0104 and HC03, with open drain outputs which do not work well with FS1000A.

So I think HCt125 is a better choice. (3) I read that UART is not a good protocol in this type a application, and NRZ/Manchester code is usually recommended. I think for my application of low speed, toy car RC switching stuff, even Manchester coding is an overkill, because speed is know for xmit and recv parties, and for short messages, no sync of clock recovering is required.

To conclude, I would start exploring with very simple procotcol, such as AFK/PWM things. Below are two references I found useful.