For an asymmetric sign pattern matrix $P$, we analyze the sign characteristic of $P$ with the help of a signed digraph in this paper. The maximal SNS-pattern matrix for a signed digraph is the maximal sign-nonsingular sub-pattern among all real matrices having the given sign pattern $P$. In this paper, SNS problems for signed digraphs are studied by converting a signed digraph $\Gamma$ into a signed bipartite graph $G(U,V)$. We propose the algorithms for searching for a sub-signed bipartite graph $G(U',V')$ with the maximum perfect matching $M'$ corresponding to every set of disjoint $M'$-interlacing cycles, which contain an even number of $M'$-interlacing $e$-cycles. The maximal SNS-pattern for a signed digraph is obtained according to algorithms.