  One-dimensional topological edge modes are usually studied considering the
interface between two different semi infinite periodic crystals (PCs) with
inverted band structure around the Dirac point. Here we consider the case where
the two PCs are finite, constituting an open scattering system, and we study
the influence of the size of this finite structure on the interface mode by
inspecting the complex resonances. First we show the complex resonance
distribution corresponding to the band inversion around the Dirac point.
Perturbations from the Dirac point display the emergence of the localized
interface mode. We also report on a remarkable robustness of the interface mode
as the system size varies which persists even for the smallest possible size.
