\begin{tikzpicture}[scale=1]
\tikzstyle{every node}=[draw, circle, fill=white, inner sep=0pt,minimum size=0pt]
\draw (0,0) node[minimum size=0.6cm](in){in}--(1,0) node{}--(2,0) node[minimum size=0.5cm](b){b}--(5,0) node[minimum size=0.5cm](e){e}--(6,0) node{}--(7,0) node[minimum size=0.6cm](out){out};
%First block of 3 things
\draw (1,0) node{}--(1,1) node{}--(2,1) node[minimum size=0.5cm](a){a}--(e);
\draw (1,0) node{}--(1,-1) node{}--(2,-1) node[minimum size=0.5cm](c){c}--(e);
%Second block of 3 things