set baudList to {1200, 2400, 4800, 4800, 9600, 19200, 38400, 57600, 115200, 230400}

set baudRate to (choose from list baudList default items {38400})

tell application "Terminal"

    set serialDevices to (do shell script "ls /dev/cu*")

    set theDeviceList to (paragraphs of serialDevices) as list

    set theDevice to (choose from list theDeviceList)

    do script "screen " & theDevice & " " & baudRate

    display dialog "To quit you terminal session type <ctrl-a> then <ctrl-\\>"

end tell