I would try setting Switch 5 to ON.
You will need to cycle power after changing the switch.
This should reset the comm parameters to default: 9600,7,E,2.
If you cannot connect, try 2400,7,E,2.
Not sure how old your CPU is. Original CPU default was 2400.
Later models improved to 9600.
If you get connected, record the values in DM6645, 6646, 6650, 6651. This will let you figure out the settings for each port.
Hope this will help.