  Propagation failure (pinning) of traveling waves is studied in a discrete
scalar reaction-diffusion equation with a piecewise linear, bistable reaction
function. The critical points of the pinning transition, and the wavefront
profile at the onset of propagation are calculated exactly. The scaling of the
wave speed near the transition, and the leading corrections to the front shape
are also determined. We find that the speed vanishes logarithmically close to
the critical point, thus the model belongs to a different universality class
than the standard Nagumo model, defined with a smooth, polynomial reaction
function.
