How to Link DOS to Ubuntu in Norton Commander with a nullmodem cable

How to use "Link" in Norton Commander between an old DOS computer and an Ubuntu computer with Norton commander inside a DOSBox.

All was tested on DOS 6.22 with Norton Commander 5.5 running on one side and Ubuntu 18.04.2 LTS  with Norton Commander 5.5 running in a DOSBox 0.74 on the other side of a (serial) nullmodem cable.

• Use a (serial) nullmodem cable on both COM1 ports.
• Set the COM1 port on both computers to the same speed.(9600 is reliable on old computers) On ubuntu set the speed with "sudo stty -F /dev/ttyS0 9600". Check the speed with "stty -F /dev/ttyS0".
• To check if the cable is going into the correct connector/port attach an old (Microsoft) serial mouse to the port, it will not work. Type in terminal "sudo inputattach --microsoft /dev/ttyS0". If you can move the pointer with the serial mouse the port is working correct and it is ttyS0. This is not a solution for a serial mouse but works fine for this test.
• To be able to create directories in DOSBox on Ubuntu start DOSBox manually in a terminal with sudo.
• Open DOSBox and configure "serial1=directserial realport:ttyS0". You can do this by typing " serial1 directserial realport:ttyS0" (This is confusing because you are using a cable called nullmodem and there is also the (wrong) option to go for "serial1=nullmodem")
• Check the setting at the dosbox command prompt with "serial1"
• Start Norton Commander 5.5 on both computers and open the menu with F9 and open the "Left" or "Right" panel, scroll down and open "Link".

Screenshot of Norton Commander 5.5 in DOSBox on Ubuntu 18.04.2 LTS

• Choose Master on one side and Slave on the other side and press Link on both sides.
Now the master computer has a panel that has files on the other computer and files can be copied from one computer to the other and back.