\documentclass{beamer}
\usetheme{Boadilla}
\usepackage{tikz}
\usetikzlibrary{shapes, arrows, arrows.meta, positioning, calc}
\begin{document}
\begin{frame}{Block Diagrams}
\begin{tikzpicture}[
block/.style = {draw, fill=white, rectangle, minimum height=3em, minimum width=3em},
scalar/.style= {draw, fill=white, circle, node distance=1cm, node contents=$a$},
sum/.style= {draw, fill=white, circle, node distance=1cm, node contents=$+$, scale = 0.7},
]
% 1st block diagram
\coordinate (input) at (0,0);
\node (s1) [scalar, right = 1cm of input]{};
\node (f1) [block, right = 1cm of s1]{$F$};
\node (out1) [right = 1cm of f1]{};
\draw [->] (input) to node[above]{$x$}(s1);
\draw [->] (s1) to (f1);
\draw [->] (f1) to node[above]{$y$}(out1);
% 3rd block diagram
\node (f3) [block, below = 1.0cm of f1]{$F$};
\node (out3) [right = 1cm of f3]{};
\draw [->] (f3) to node[above]{$y$}(out3);
\node (sum1) [sum, left = 0.5cm of f3]{};
\draw [->] (sum1) to (f3);
\node (c1) [above left = 0.5 and 0.5 of sum1]{};
\node (c2) [below left = 0.5 and 0.5 of sum1]{};
\node (in3) [left = 1cm of c1]{};
\node (in4) [left = 1cm of c2]{};
\draw [->] (c1) to (sum1);
\draw [->] (c2) to (sum1);
% These two lines do not connect as expected
\draw [-] (in3) to node[above]{$x1$}(c1);
\draw [-] (in4) to node[above]{$x2$}(c2);
\end{tikzpicture}
\end{frame}
\end{document}
решение1
Узлы имеют размеры, координаты — нет, см.TikZ: разница между \node и \coordinate?.
\documentclass{beamer}
\usetheme{Boadilla}
\usepackage{tikz}
\usetikzlibrary{shapes, arrows, arrows.meta, positioning, calc}
\begin{document}
\begin{frame}{Block Diagrams}
\begin{tikzpicture}[
block/.style = {draw, fill=white, rectangle, minimum height=3em, minimum width=3em},
scalar/.style= {draw, fill=white, circle, node distance=1cm, node contents=$a$},
sum/.style= {draw, fill=white, circle, node distance=1cm, node contents=$+$, scale = 0.7},
]
% 1st block diagram
\coordinate (input) at (0,0);
\node (s1) [scalar, right = 1cm of input];
\node (f1) [block, right = 1cm of s1]{$F$};
\node (out1) [right = 1cm of f1]{};
\draw [->] (input) to node[above]{$x$}(s1);
\draw [->] (s1) to (f1);
\draw [->] (f1) to node[above]{$y$}(out1);
% 3rd block diagram
\node (f3) [block, below = 1.0cm of f1]{$F$};
\node (out3) [right = 1cm of f3]{};
\draw [->] (f3) to node[above]{$y$}(out3);
\node (sum1) [sum, left = 0.5cm of f3];
\draw [->] (sum1) to (f3);
\coordinate (c1) at ([shift={(-.5,.5)}]sum1);
\coordinate (c2) at ([shift={(-.5,-.5)}]sum1);
\node (in3) [left = 1cm of c1]{};
\node (in4) [left = 1cm of c2]{};
\draw [->] (c1) to (sum1);
\draw [->] (c2) to (sum1);
\draw [-] (in3) to node[above]{$x1$}(c1);
\draw [-] (in4) to node[above]{$x2$}(c2);
\end{tikzpicture}
\end{frame}
\end{document}
Я хотел бы показать альтернативное решение с матрицей TikZ:
\documentclass{beamer}
\usetheme{Boadilla}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{frame}{My proposal}
\begin{tikzpicture}[
block/.style = {draw, rectangle, minimum height=3em, minimum width=3em},
scalar/.style= {draw, circle},
sum/.style= {scale = 0.7},
inout/.style= {inner sep = 0pt},
]
\matrix[
matrix of math nodes,
ampersand replacement=\&,
column sep = 1cm,
nodes={anchor=center},
column 1/.style={nodes={inout}},
column 2/.style={nodes={scalar}},
column 3/.style={nodes={block}},
column 4/.style={nodes={inout}},
]{
% 1st block diagram
|[name=in1]|{} \&[.1cm] |[name=s1]|{a} \&
|[name=f1]|{F} \&
|[name=out1]|{}\\[1cm]
% 3rd block diagram
|[name=in3up]|{} \\[-.1cm]
\& |[name=sum3,sum]|{+} \&
|[name=f3]|{F} \&
|[name=out3]|{}\\[-.1cm]
|[name=in3down]|{} \\
};
% connections of the 1st block diagram
\draw[->] (in1) -- node[above]{$x$} (s1);
\draw[->] (s1) -- (f1);
\draw[->] (f1) -- node[above]{$y$} (out1);
% connections of the 3rd block diagram
\draw [->] (in3up) -- node[above]{$x_1$} ++(1,0) -- (sum3);
\draw [->] (in3down) -- node[above]{$x_2$} ++(1,0) -- (sum3);
\draw [->] (sum3) -- (f3);
\draw[->] (f3) -- node[above]{$y$} (out3);
\end{tikzpicture}
\end{frame}
\end{document}
решение2
Разновидность кода OP. Учитывая предложение @CarLaTeX использовать \coordinateвместо \node, с использованием библиотеки quotes и более последовательным использованием библиотеки positioning, MWE может быть:
\documentclass{beamer}
\usetheme{Boadilla}
\usepackage{tikz}
\usetikzlibrary{arrows.meta, positioning, quotes}
\begin{document}
\begin{frame}
\frametitle{Block Diagrams}
\centering % if needed
\begin{tikzpicture}[auto,
node distance = 10mm and 10 mm,
base/.style = {draw, fill=white, minimum size=5mm, inner sep=0pt},
block/.style = {base, minimum size=3em},
scalar/.style = {base, circle, node contents=$a$},
sum/.style = {base, circle, node contents=$+$},
every edge/.style = {draw, -Straight Barb}
]
% 1st block diagram
\coordinate (in1);
\node (s1) [scalar, right = of in1];
\node (f1) [block, right = of s1] {$F$};
\coordinate[right=of f1] (out1);
%
\draw (in1) edge ["$x$"] (s1)
(s1) edge (f1)
(f1) edge ["$y$"] (out1);
% 3rd block diagram
\node (f3) [block, below = of f1] {$F$};
\coordinate[right=of f3] (out3);
\scoped[node distance=5mm and 5mm]
{
\node (sum) [sum, left=of f3];
\coordinate[above left=of sum] (c1);
\coordinate[below left=of sum] (c2);
}
\coordinate[left=of c1] (in3);
\coordinate[left=of c2] (in4);
%
\draw (in3) to["$x1$"] (c1) edge (sum);
\draw (in4) to["$x2$"] (c2) edge (sum);
\draw (sum) edge (f3)
(f3) edge["$y$"] (out3);
\end{tikzpicture}
\end{frame}
\end{document}
