If you are getting a Fadal control error "PARAMETER ERROR" this indicates that the user entered an invalid Fadal command. This is not a Calmotion error.
What is most likely happening is the user is pressing another key after the + For example the user is probably pressing the <enter> button after the Calmotion + command.
Make sure when entering a Calmotion + command that the + is the last key pressed. Wait for the Calmotion device to respond to the command with an @ symbol.
To get a directory:
Do not press anything button after the + command.
Here is a video showing the steps to get a response: