Discrete time physical system modeling using latched input response for physically cross-coupled state variable models including variable sampling capability. Classical, fixed sampling impulse modeling using Laplace/Ztransform methods. System response calculations, direct design methods, root locus design methods, command feedforward tracking methods, disturbance input decoupling design methods, and cascade control methods including multi-rate sampling systems. Control processor interfaces, feedback sensor & interface issues, open and closed loop observers, controller design including correct consideration of computational delays, discrete time system identification methods, and structured approaches for sequential logic design.