Hi,
  Now I'm using verilog-a to do some research on modeling kinematic structures. We all know that ddt or idt is accepted in verilog-a, such like the following sentences:
  Pos(v)<+ddt(x);

 But if Fdrv is from a source outside my module and i want to output the x in the following differential equation, how can i?

 a*x''+b*x'+cx=Fdrv

  i've written these codes,
  .......(includings)
  module test(Fdrv,x);
      inout Fdrv,x;
      kinematic Fdrv,x;
      kinematic v; //temp. var. to hold ddt(x)
      parameter real a=xxx, b=xxx, c=xxx; // assume a,b,c are properly defined      
              analog begin
                   Pos(v)<+ddt(x);
                   F(Fdrv)<+a*ddt(Pos(v))
                                 +b*Pos(v)
                                 +c*Pos(x);
               end
  endmodule

   if i input x to module test, i will get proper output of Fdrv, but if i want to input Fdrv and get the
proper value of x, which means i want to solve this differential equation, it never works.
   How can i solve this problem? thank you very very much....