\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}

\usepackage{tikz}
\usetikzlibrary{arrows,positioning, calc,lindenmayersystems,decorations.pathmorphing,intersections} 
\tikzstyle{resource}= [draw,minimum size=16pt,inner sep=0pt]
\tikzstyle{process} = [draw,minimum size=16pt,inner sep=0pt,circle]
\tikzstyle{allocation} = [->,thick,arrows={-latex}]

\begin{document}
\begin{preview}
\begin{tikzpicture}[scale=2]
    \node (r1)[resource] at (0,2) {$R_1$};
    \node (r2)[resource] at (1,2) {$R_2$};
    \node (r3)[resource] at (2,2) {$R_3$};
    \node (p1)[process]  at (0,1) {$P_1$};
    \node (p2)[process]  at (1,1) {$P_2$};
    \node (p3)[process]  at (2,1) {$P_3$};
    \node (r4)[resource] at (0,0) {$R_4$};

    \draw[allocation] (r1) -- (p1);
    \draw[allocation] (r2) -- (p2);
    \draw[allocation] (r3) -- (p2);
    \draw[allocation] (p3) -- (r3);
\end{tikzpicture}
\end{preview}
\end{document}
