Question
How can Rpi4B talk to duplex BUS UART servo LX-224HV?
Answer
Part B - Rpi4B UART to LX-224HV Simplex Serial Ciruit Design V0.1
Next step is to let Rpi to send commands to LX-224 and see if is replies back anything. The OP reports nothing comes back. So I will repeat his commands and see if I have better luck.
The Op is sending the following commands. I think I better read the servo command manual to make sure the commands are OK and will not fry the servo.
0x55 0x55: transmission start tag. 0xFE: Id of the target servo, 254 means broadcast 0x04: Length of the block, including this length to ending checksum 0x0E: 14 is Read_ID, asking the servo for it ID. 0x01: default Id (not used?) 0xEE: Checksum ~(254+4+14+1) = ~(17) = 238 Part A - LX224HX Troubleshooting Using BusLink V2.4
I am using the HiWonder Bus Linker V2.4 to display the send receive signals when the LX-224 UART/Bus servo is set to motor mode and move continuously at about 700 rpm.
References
(1) LewanSoul Bus Servo Communication ProtocolLewanSoul Bus Servo Communication Protocol
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Appendices
Appendix A - The OP's test setup
Appendix B - HiWonder LX-224 BusServoController setup
Appendix C - HiWonder LX-224 BusLink V2.4 Setup
Appendix D - TSX0104 Level Shifter for implementing LX224 Simplex UART/BUS Servo Bidirectional Control
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Not yet completed answer. Stay tuned, ...