I am not able to send data from the NANO-PC-T4 on ttyS4.
I am able to receive the data.
Setup
Nano-pc-t4 ----> USB to Serial --> PC
(40 pin header) (CP1201)

(PC-T4 Tx Pin-19 ------------> Rx of USB to Serial)
(PC-T4 Tx Pin-21 ------------> Tx of USB to Serial)

What I type in the PC terminal is seen in the NANO-PC-T4.
What I type in NANO-PC-T4 is not seen in the PC terminal.
Is there any pull-up needed on the TX ?

I could see that /proc/tty/driver/serial , UART-4 counters are incremented.

4: uart:16550A mmio:0xFF370000 irq:40 tx:21 rx:20

Can you tell me how to debug this ?