Two different unpowered behaviors
A normally open, or momentary, multi-position switch disconnects the common RF input from the selectable output ports when no actuator is energized. A failsafe switch instead returns to a predetermined connected position when power is removed.
Momentary normally open operation
Applying voltage to the selected actuator closes the corresponding RF path and maintaining the command holds that position. Removing the command returns the path to the open condition. This architecture can be useful when the desired safe state is no connection between the common port and the outputs.
Failsafe operation
A failsafe mechanism provides a specific RF connection with no actuator power. It is therefore appropriate when a receiver, load, antenna, calibration path or other circuit must remain selected after control power disappears.
System design implications
The two designs should not be treated as interchangeable. Consider the required state during startup, shutdown, cable faults and power interruption; whether continuous hold current is acceptable; and whether more than one actuator command could be asserted by the controller.
Selection example
If an SP6T test system should disconnect all devices under test whenever the controller is off, a momentary normally open topology may be attractive. If the same system must automatically reconnect a calibration standard after power loss, a failsafe topology may be preferable.
Key takeaway
Normally open disconnects the RF paths when unpowered; failsafe returns to a predetermined connected position.