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Components 
External Port
External ports represent the sources and sinks of your design, i.e., they mark the inputs and outputs of your system. They can be added to your design by dragging them from the block selection tree and dropping them inside the schematic view or by simply using the shortcut 'p' (for 'port') there.
Inside the schematic
view you must connect the external port with an internal port
of a block. Therefore, choose 
Home:
Components Connector
or use the shortcut
'c' (for 'connect) to activate the link mode. Then
perform two mouse clicks — one at each port — to establish
the connection.
An external port is characterized by a number that it is automatically assigned to it after its insertion, but can be modified by the user. It is further characterized by the mode to consider and an impedance that the port may be associated with. This characteristic impedance may be chosen as a fixed impedance or as dependent on the input impedance of the block that it is connected with. All these properties can be modified within the External Port Properties - General dialog box.
See also