  Maximum Distance Separable (MDS) convolutional codes are cha- racterized
through the property that the free distance meets the generalized Singleton
bound. The existence of free MDS convolutional codes over Z p r was recently
discovered in [26] via the Hensel lift of a cyclic code. In this paper we
further investigate this important class of convolutional codes over Z p r from
a new perspective. We introduce the notions of p-standard form and r- optimal
parameters to derive a novel upper bound of Singleton type on the free
distance. Moreover, we present a constructive method for building general (non
necessarily free) MDS convolutional codes over Z p r for any given set of
parameters.
