fix #909

Author: baggepinnen

lib/ControlSystemsBase/src/discrete.jl

@@ -143,6 +143,9 @@ This method of translation is useful when analyzing hybrid continuous/discrete s
 The resulting system will be be a static system in feedback with pure delays. When `method = :causal`, the delays will be positive, resulting in a causal system that can be simulated in the time domain. When `method = :acausal`, the delays will be negative, resulting in an acausal system that **can not** be simulated in the time domain. The acausal translation results in a smaller system with half as many delay elements in the feedback path.
 """
 function d2c_exact(sys::AbstractStateSpace{<:Discrete}, method=:causal)
+    if sys.nx == 0
+        return DelayLtiSystem(d2c(sys))
+    end
     T = sys.Ts
     A,B,C,D = ssdata(sys)
     if method === :acausal

lib/ControlSystemsBase/test/test_discrete.jl

@@ -164,6 +164,13 @@ pd = c2d_poly2poly(A, 0.1)
     # rd = step(sys, 0:10)
     # rc = step(sysc_causal, 0:10)
     # @test rd.y ≈ rc.y atol = 1e-8
+
+    sys = ss(1,1)
+    sysc = d2c_exact(sys)
+    sysc2 = DelayLtiSystem(ss(1))
+    @test sysc.P.P ≈ sysc2.P.P
+    @test sysc.P.ny ≈ sysc2.P.ny
+    @test sysc.Tau ≈ sysc2.Tau
 end