Procedure HexToByte(Hex:String; Var Decimal:Byte; Var Code:Integer);

    The string Hex must  be  1  or  2  digits  in  length.   The value is
    returned in  Decimal  unless  an  invalid  character  is  detected in
    Hex.   If  an   illegal   character   is   detected,  Code  indicates
    the position of this error.  If  Code  > 0 then Decimal is undefined.
    See HexToLongInt for more details.