@RyanFikri Ah, let me see. You don't seem to understand the meaning of loopback. Now let me give the steps in more details. (1) Put away Rpi, (2) Put away servo, (3) Only use Win10 RealTerm, (4) Connect USB to serial cable to WinPC USB socket, (5) Connect the cable's TxD to RxD (This way output to TxD will input to RxD, that is the meaning of loop back or echo), (5) Type/send "abc123" + <newline> at RealTerm. You should see "abc123" echoing back to RealTerm.