\documentclass[tikz]{standalone}

\usepackage{tikz}
\usetikzlibrary{arrows,shapes,positioning,decorations,decorations.markings,shadows}

\tikzstyle{block} = [rectangle, draw, fill=white, text centered, 
text width = 3em, minimum height = 1.5em]
\tikzstyle{point} = [circle, fill=green!80, minimum height=0.5em]
\tikzstyle{line} = [draw, -latex']

\begin{document}
\begin{tikzpicture}[]
  \node [block] (F) at (1.5, 0) {};
  \node [point] at (1.5, 0) {};

  \node [] (L) at (0,0) {{\ttfamily I}};
  \node [] (R) at (3,0) {{\ttfamily O}};

  \node [above of=F, node distance=1.5em] {{\ttfamily Function}};

  \path[line] (F) -- (R);
  \path[line] (L) -- (F);
\end{tikzpicture}
\end{document}