Sony MSW-900 Operation Manual page 222

To see the image of the return video signal on the viewfinder
screen
Press the RET switch on the lens. The image of the return video signal
input to the GENLOCK IN connector is displayed on the viewfinder
screen while you are pressing the RETURN switch on the lens.
When the RETURN VIDEO function is assigned to the ASSIGN 1
switch, you can see the image of the return video signal on the
viewfinder screen while you are pressing the ASSIGN 1 switch even if
4
RETURN VIDEO is set to OFF on the GENLOCK page of the
MAINTENANCE menu.
Note
When any signal is not input to the GENLOCK IN connector, the image
is not changed even if you press the RET switch on the lens.
However, the image is switched while you are pressing the ASSIGN 1
switch to which the RETURN VIDEO function is assigned, even if any
signal is not input to the GENLOCK IN connector.
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Chapter 4 Adjustments and Settings for Recording