This disclosure involves a method of controlling a safety critical control device, the method comprising: sending user inputs to a first state machine, identifying user inputs by the first state machine, determining the correct state to communicate to a second state machine, the correct state being determined by selecting one state of a plurality of states depending on the user inputs, communicating the correct state to a second state machine through a control bus, and determining the correct state for the second state machine based on communication from the control bus.