Burst Electronics VDS-3 - Manual (Page 4)

Forcing the B Input
The presence of the B input can be confirmed with the “Force B”
feature. With the VDS and the VSA, ground the appropriate pin of
the MiniDIN connector. This will force the unit to select the B
input. See the diagram at the end of the manual for the connector
pinout.
The VSA also offers a front panel test switch to check for the
presence of the B input.
The VDS-3 also has a front panel switch to force B. The remote
Force B and Tally outputs are factory options for the VDS-3.
3 Video Detector