Anritsu 681XXC - Manual (Page 112)

External Source Input Connector Selection
If you have selected External to use a modulating
signal from an external source, then when you press
More the menu (below) is displayed.
Press Front/Rear to select the front or rear panel
IN connector. The Square Wave status display
shows your selection as Source.
Press Previous Menu to return to the initial Square
Wave Status Menu display.
681XXC OM 3-79
LOCAL (FRONT SIGNAL
PANEL) OPERATION MODULATION