The IsSystem command checks the validity of the system object.

If sys is a valid system object, return true. If sys contains errors, return false.

To determine the problem with a system that fails, use DynamicSystems[Verify].

with( DynamicSystems ):

sys1 := NewSystem():

PrintSystem(sys1);

$\left[\begin{array}{l}\mathbf{Algebraic\; Equation}\\ \mathrm{continuous}\\ \mathrm{1\; output(s);\; 1\; input(s)}\\ \mathrm{inputvariable}\=\left[u\left(t\right)\right]\\ \mathrm{outputvariable}\=\left[y\left(t\right)\right]\\ \mathrm{ae}\=\left[y\left(t\right)=u\left(t\right)\right]\end{array}\right.$

IsSystem(sys1);

$\mathrm{true}$

sys2 := NewSystem(s/(s^3+5*s^2+7*s+6)):

sys2:-tf := <<exp(s)/(s^3+5*s^2+7*s+6)>>:

PrintSystem(sys2);

$\left[\begin{array}{l}\mathbf{Transfer\; Function}\\ \mathrm{continuous}\\ \mathrm{1\; output(s);\; 1\; input(s)}\\ \mathrm{inputvariable}\&equals;\left[\mathrm{u1}\left(s\right)\right]\\ \mathrm{outputvariable}\&equals;\left[\mathrm{y1}\left(s\right)\right]\\ {\mathrm{tf}}_{1,1}\&equals;\frac{{\&ExponentialE;}^s}{s^3+5s^2+7s+6}\end{array}\right.$

IsSystem(sys2);

$\mathrm{true}$

tf_mimo_z := Matrix([[1/z^2, z^2/(z^3+5*z^2+7*z+6)], [1/z , c/(z^2+a*z+b)]]):

sys3 := NewSystem(tf_mimo_z, discrete, sampletime=0.001, systemname="Sample discrete MIMO system"):

PrintSystem(sys3);

$\left[\begin{array}{l}\mathbf{Transfer\; Function}\\ \mathrm{discrete;\; sampletime\; =\; .1e-2}\\ \mathrm{systemname}\&equals;\mathrm{Sample\; discrete\; MIMO\; system}\\ \mathrm{2\; output(s);\; 2\; input(s)}\\ \mathrm{inputvariable}\&equals;\left[\mathrm{u1}\left(z\right)\&comma;\mathrm{u2}\left(z\right)\right]\\ \mathrm{outputvariable}\&equals;\left[\mathrm{y1}\left(z\right)\&comma;\mathrm{y2}\left(z\right)\right]\\ {\mathrm{tf}}_{1,1}\&equals;\frac{1}{z^2}\\ {\mathrm{tf}}_{2,1}\&equals;\frac{1}{z}\\ {\mathrm{tf}}_{1,2}\&equals;\frac{z^2}{z^3+5z^2+7z+6}\\ {\mathrm{tf}}_{2,2}\&equals;\frac{c}{z^2+az+b}\end{array}\right.$

IsSystem(sys3);

$\mathrm{true}$