function [Ad, Bd, Cd, Dd] = c2dm(sys, t, ~)
%C2D continuous system to discrete time system conversion
% Use an arbitrary third argument for FOH approximation matrixes.

% by H. Shakouri G.

 [a, b, c, d] = ssdata(sys);
  n = size(a,1);
  m = size(b,2);
  s = expm([[a,b]*t; zeros(m, n+m)]);
  Ad = s(1:n, 1:n);
  Bd = s(1:n,n+1:n+m);
  Cd = c;
  Dd = d;
  if nargin==3
    AI = Ad - eye(size(a));
    a1 = inv(a);
    G = a1*AI*b;
    H = (a1*a1*AI/t-a1)*b;
    Bd = AI*H+G;
    Dd = d + c*H; 
  endif
  if nargout<=1, Ad = ss(Ad, Bd, Cd, Dd); end
endfunction