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uses
  SysUtils;

var s:String;

  ...
  s:=IntToHex(255, 2);
  ...                    

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uses
  Math;

function HexCharToWert(CharWert:Char):Integer;
begin
  case CharWert of
    'A'..'F':Result:=Ord(CharWert)-65+10;
    else Result:=StrToInt(CharWert);
  end; // case
end; // HexCharToWert


function HexToInt(const HexWert:String):Integer;
var i:Integer;
    s:String;
begin
  Result:=0;
  s:=UpperCase(HexWert);
  for i:=1 to Length(s) do if (Ord(s[i]) in [0..47, 58..64, 71..255]) then
  begin
    Result:=-1;
    Break;
  end; // for i
  if (Result=0) then for i:=0 to Length(s)-1 do
    Result:=Result+(HexCharToWert(s[Length(s)-i])*Round(Power(16, i)));
end; // HexToInt

...
...
var s:String;

  ...
  s:=IntToStr(HexToInt('FF'));
  ...                    

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zahl := StrToInt('$'+hexwert);