@Trumany I just came across this thread because I am having the same problem as @locar_chang - I am connecting a device (Impinj RS1000 RFID module) via UART (J41 / ttyTHS1) to my nano. The RX on the nano is working - I can read data from the RS1000 just fine - it operates at 3.3v. However, TX is not working - the RS1000 won’t receive any data from the nano. I don’t have an oscilloscope handy to confirm it is in fact putting out <1.8v, but I have a strong suspicion this is the issue. I noticed that the Rpu of the UART interface on the RS1000 device is min/typ/max 25/40/55 Kohm (see p.11 of the attached spec), so I’m wondering if the typical “40” is not sufficient? If so do I need to put additional resistance in series on the nano’s TX line for this to work?