\begingroup
\catcode94=7 % circumflex
\catcode0=11 % null byte
\def\makeletterchar#1{%
	\begingroup
	\lccode0=#1% space isn't needed because \lowercase terminates
	\lowercase{\endgroup\gdef\letterchar{^^@}}%
}

% prepare ONE-BYTE conversion [0-255] -> binary character:
\count0=0
\countdef\counter=0
\loop\ifnum\counter<256
	\makeletterchar{\counter}%
	\expandafter\xdef\csname pgfp@bin@\the\counter\endcsname{\letterchar}
	\advance\counter by1
\repeat
\ifnum\counter=256
\else
	\pgfplots@error{Internal logic Error: initialisation failed. I have only \the\counter\space bytes initialised.}
\fi
\endgroup

\endinput
