  The framework of port-Hamiltonian (pH) systems is a powerful and broadly
applicable modeling paradigm. In this paper, we extend the scope of pH systems
to time-delay systems. Our definition of a delay pH system is motivated by
investigating the Kalman-Yakubovich-Popov inequality on the corresponding
infinite-dimensional operator equation. Moreover, we show that delay pH systems
are passive and closed under interconnection. We describe an explicit way to
construct a Lyapunov-Krasovskii functional and discuss implications for delayed
feedback.
