File:Soi diag ex.svg

\documentclass[tikz]{standalone}

\usetikzlibrary{arrows}
\tikzstyle{dspsquare}=[rectangle, thick, draw=black, fill=gray!10, minimum size=1cm]
\tikzstyle{dspcircle}=[circle, thick, draw=black, fill=gray!10, minimum size=1cm]
\tikzstyle{dspjoint}=[fill,draw=black,shape=circle,minimum size=3pt, inner sep=0]

\begin{document}

\begin{tikzpicture}[>=triangle 45]
\matrix(m1) [row sep=2.5mm, column sep=5mm]
{
	\node					(m00)	{$x(n)$};		        &
	\node[dspjoint]				(m01)	{};				&
	\node[dspsquare]			(m02)	{$z^{-1}$};		        &
	\node					(m03)	{};			       \\
	%------------------------------------------------------------------------------
	\node					(m10) 	{};				&
	\node[dspsquare]			(m11)	{0,5};		                &
	\node					(m12)	{};				&
	\node[dspsquare]			(m13)	{0,5};		               \\
	%------------------------------------------------------------------------------
	\node					(m20) 	{};				&
	\node					(m21)	{};				&
	\node					(m22)	{};				&
	\node					(m23)	{};			       \\		
	%------------------------------------------------------------------------------
	\node					(m30) 	{};				&
	\node					(m31)	{};				&
	\node[dspcircle]			(m32)	{$+$};			        &
	\node					(m33)	{};			       \\	
	%------------------------------------------------------------------------------
	\node					(m40) 	{};				&
	\node					(m41)	{};				&
	\node					(m42)	{};				&
	\node					(m43)	{$y(n)$};		       \\			
};

\draw[->] 	(m00) -- (m01) -- (m02);
\draw[->]	(m02) -| node[auto] {$x(n-1)$} (m13);
\draw[->]	(m01) -- (m11);

\draw[->]	(m11) |-  (m32);
\draw[->]	(m13) |-  (m32);

\draw[->]	(m32) |- (m43);
\end{tikzpicture}

\end{document}