Distance between nodes in relative positioning in tikz

Question

I'm creating a block diagram and I'm using relative positioning. My code is here. The specified distance between the blocks is 2 cm. But this is the distance between the center of block1 and the left border of block2. It's not very convenient. Tell me please:

1) How can I set distance between centers of blocks?

2) It’s possible set block position by setting length of connecting arrow?

\documentclass[tikz,14pt,border=5pt]{standalone}
\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}

\tikzset{font={\fontsize{14pt}{17}\selectfont}}
\usetikzlibrary{shapes,arrows}
%\usetikzlibrary{positioning}

\begin{document}

\begin{tikzpicture}[auto,>=latex',
 block/.style = {draw, shape=rectangle, minimum height=3em, minimum width=3em, line width=1pt},
branch/.style={fill,shape=circle,minimum size=5pt,inner sep=0pt}]

\draw[help lines] (0,0) grid (5,-2);    

%Positioning Blocks  
\node [block] (block1) at (0,0) {BLOCK1};  
\node [block] (block2) [right = 2.0cm] at (block1) {BLOCK2};

%Conecting Blocks
\begin{scope}[line width=1pt, >=triangle 45]      
\draw[->] (block1) -- (block2);        
\end{scope}

\draw[fill,red] (0,0) circle [radius=0.05];
\draw[fill,red] (2,0) circle [radius=0.05];
\draw[<->,red] (0,-1.5) -- node [above] {2cm} (2,-1.5);  

\end{tikzpicture}

\end{document}

Answer 1

Welcome! You commented out \usetikzlibrary{positioning}. If you use it, your problem is solved. You can then say

\node [block] (block2) [right = 2.0cm of block1] {BLOCK2}; 

The arrow length can be adjusted with shorten.

\documentclass[tikz,14pt,border=5pt]{standalone}
\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}

\tikzset{font={\fontsize{14pt}{17}\selectfont}}
\usetikzlibrary{arrows,positioning}

\begin{document}
\begin{tikzpicture}[auto,>=latex',
 block/.style = {draw, shape=rectangle, minimum height=3em, minimum width=3em, line width=1pt},
branch/.style={fill,shape=circle,minimum size=5pt,inner sep=0pt}]

\draw[help lines] (0,0) grid (5,-2);    

%Positioning Blocks  
\node [block] (block1) at (0,0) {BLOCK1};  
\node [block] (block2) [right = 2.0cm of block1] {BLOCK2};

%Conecting Blocks
\begin{scope}[line width=1pt, >=triangle 45,shorten >=2pt, shorten <=1pt]      
\draw[->] (block1) -- (block2);        
\end{scope}

\end{tikzpicture}

\end{document}

Please note that arrows.meta is superior to the arrows library. Note also that if you want to draw more nodes along this direction, the chains library can be handy.

ADDENDUM: As Torbjørn T. points out, I did not read the question carefully enough. If you want to specify the distance between the centers of the block, add the on grid option.

\documentclass[tikz,14pt,border=5pt]{standalone}
\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}

\tikzset{font={\fontsize{14pt}{17}\selectfont}}
\usetikzlibrary{arrows,positioning}

\begin{document}
\begin{tikzpicture}[auto,>=latex',
 block/.style = {draw, shape=rectangle, minimum height=3em, minimum width=3em, line width=1pt},
branch/.style={fill,shape=circle,minimum size=5pt,inner sep=0pt},
on grid]

\draw[help lines] (0,0) grid (5,-2);    

%Positioning Blocks  
\node [block] (block1) at (0,0) {BLOCK1};  
\node [block] (block2) [right = 3.0cm of block1] {BLOCK2};

%Conecting Blocks
\begin{scope}[line width=1pt, >=triangle 45,shorten >=2pt, shorten <=1pt]      
\draw[->] (block1) -- (block2);        
\end{scope}

\end{tikzpicture}

\end{document}