  The standard boundary state of a topological insulator in 3+1 dimensions has
gapless charged fermions. We present model systems that reproduce this standard
gapless boundary state in one phase, but also have gapped phases with
topological order. Our models are weakly coupled and all the dynamics is
explicit. We rederive some known boundary states of topological insulators and
construct new ones. Consistency with the standard spin/charge relation of
condensed matter physics places a nontrivial constraint on models.
