A human machine interface includes a display screen coupled to a processing device. The display screen is disposed on a case and displays an image frame. In the processing unit, a comparing module compares a first trigger signal stored in a storage unit with a monitored signal transmitted from a signal source. A notification signal is generated when the first trigger signal is detected appearing in the monitored signal. A control module receives the notification signal and generates a first alert signal to display in the image frame or on the case.