Controlling a DiscreteDevice object via outputs

You can control (command) the passive, first active, and second active states of the discrete device via the Cmd attribute. The transition and fault states are not commandable. When a DiscreteDevice object is commanded to a new state, it sets an appropriate combination of discrete outputs for that state.

For example, you might map the first active state of "Open" to a discrete device output reference that will turn a valve to the open position. When you set the Cmd attribute to "Open," the valve will be "commanded" to be opened. If inputs (feedback) are configured for the DiscreteDevice object, the object could then detect when the valve is actually open or closed.

Commanding a discrete device to a new state does not directly change the PV unless you have the PVMode set to "Simulate."

Controller-oriented features for the DiscreteDevice object include:

• A configurable control mode. The control mode (Manual or Cascade) determines what types of writes are allowed to the Cmd value within the system. Manual indicates that only users (operators) are allowed to requst a change to Cmd, whereas Cascade indicates that only scripts or other supervisory objects are allowed to request a change to Cmd.

• Control tracking. A Boolean control track flag can be read from an external device or object. When tracking is on, the external controller "owns" the value of the Cmd; the Cmd is purely read-only and is set to the vsalue of PV.