From Variscite Wiki

The default Linux release exposes two DART-SD410 UARTs. To show all the available UARTs use:

# ls /dev/ttyMSM*
The output will be:
/dev/ttyMSM0  /dev/ttyMSM1
  • /dev/ttyMSM0 is used by serial console
  • /dev/ttyMSM1 exposed as user UART on the J7 Connector.

To test the /dev/ttyMSM1 UART connect pins 2 & 3 (RX & TX) and 7 & 8 (CTS & RTS) on the J7 connector. Configure the UART to 115200 baud using rts-cts flow control

# stty -F /dev/ttyMSM1 -echo -onlcr 115200 crtscts

Start receive task in background:

# cat /dev/ttyMSM1 &

Send something to the port:

# echo "Serial Port Test" > /dev/ttyMSM1
The output will be
# Serial Port Test
The receiver printed out the message.

To stop the background receiver use

# fg
cat /dev/ttyMSM1
Press ctrl+c to exit

You can try the same scenario with pins 7 & 8 disconnected.
In this case the transmit buffer will get all the data send to /dev/ttyMSM1 and hold for handshake. The data will be printed out as soon as the pins 7 & 8 will be connected.