Which device changes the state of a controlled device in response to a command from a building automation system controller?

• A. Building automation system output
• B. Accumulator
• C. Latching relay
• D. Pulse width modulation

Answer: (A)

Controllers issue commands through their outputs, and those outputs are what drive the field devices to switch states. The building automation system output is the channel that translates the controller’s instruction into a physical action, energizing relays or actuators so the controlled device actually changes state.

An accumulator stores energy and doesn’t trigger a state change in response to commands. A latching relay is a type of actuator that can change state, but the action of changing state is carried out by the relay in response to the command delivered through the BAS output. Pulse width modulation controls the power level delivered to a device rather than producing a discrete on/off change. In this context, the device that directly enacts the command and makes the controlled device switch states is the BAS output.