StateSpaceModel[x'[t]+x[t-1]==2 u[t]+u[t-3],x[t],u[t],x[t],t]

StateSpaceModel[{{{-SystemsModelDelay[2]}},{{3+SystemsModelDelay[1]}},{{1}},{{0}}}]

TransferFunctionModel[((3+Exp[-s])/(s+Exp[-2 s])),s]

TransferFunctionModel[((3+SystemsModelDelay[1])/(s+SystemsModelDelay[2])),s]

StateSpaceModel[{{{-SystemsModelDelay[2]}},{{3+SystemsModelDelay[1]}},{{1}},{{0}}},SamplingPeriod->2]

TransferFunctionModel[((3+SystemsModelDelay[1])/(z+SystemsModelDelay[2])),z,SamplingPeriod->2]

StateSpaceModel[(TransferFunctionModel[((3+(SystemsModelDelay[1]))/(s+(SystemsModelDelay[2]))), ])]

TransferFunctionModel[(StateSpaceModel[-(SystemsModelDelay[2]), 3+(SystemsModelDelay[1]), 1, 0, 2]),z]

ToContinuousTimeModel[(StateSpaceModel[-(SystemsModelDelay[2]), 1, 3+(SystemsModelDelay[1]), 0, 2]),Method->"BilinearTransform"]

ToDiscreteTimeModel[(StateSpaceModel[-(SystemsModelDelay[1]), 2+(SystemsModelDelay[3]), 1, 0, ]),2]