  We discuss a class of linear control problems in a Hilbert space setting.
This class encompasses such diverse systems as port-Hamiltonian systems,
Maxwell's equations with boundary control or the acoustic equations with
boundary control and boundary observation. The boundary control and observation
acts on abstract boundary data spaces such that the only geometric constraint
on the underlying domain stems from requiring a closed range constraint for the
spatial operator part, a requirement which for the wave equation amounts to the
validity of a Poincare-Wirtinger-type inequality. We also address the issue of
conservativity of the control problems under consideration.
