If you are using a MS-DOS program in a Terminal Services session, you may need to change the serial COM port mapping.
You can use the change port command, which can change COM port mapping for the current session. The syntax is:
CHANGE PORT \[portx=porty | /D portx | /QUERY\] portx=porty Map port x to port y. /D portx Delete mapping for port x. /QUERY Display current mapping ports.