The faceplate shows the device feedback (running, stopped, open or closed, as reported by the device) next to the commanded state, the operating mode that determines who may command the device, and the interlock and permissives that determine whether a command is accepted. The component does not write to the plant. A command button raises a request event, the application performs the write, and the faceplate updates when the new feedback is reported. A command that cannot be requested shows the reason.
Click Start on the pump. The faceplate shows the request as pending until the application reports the feedback. The agitator is in auto mode, so it has to be switched to manual before it accepts a command.
A device under local control or behind an interlock can be stopped from the screen but not started, and a faceplate whose interactive property is not set accepts no commands. The command buttons stay readable and show the reason when pressed.