Looks like that you just can make one connection to a /dev/tty... If you try to do more you get the busy message. Try to add the option: -d -r Reattach a session and if necessary detach it first. ex: screen -d -r /dev/tty.SLAB_USBtoUART 115200 Check out more options with man screen I checked under Linux Mint. Are you using OSX?