I propose a function :

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FUNCTION F_ASCII_TO_STRING : STRING(1)

VAR_INPUT

InputVal: UINT;

END_VAR

VAR

Caracter:STRING(1);

END_VAR

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CASE InputVal OF

48:Caracter:='0';

49:Caracter:='1';

50:Caracter:='2';

51:Caracter:='3';

52:Caracter:='4';

53:Caracter:='5';

54:Caracter:='6';

55:Caracter:='7';

56:Caracter:='8';

57:Caracter:='9';

65:Caracter:='A';

66:Caracter:='B';

67:Caracter:='C';

68:Caracter:='D';

69:Caracter:='E';

70:Caracter:='F';

71:Caracter:='G';

72:Caracter:='H';

73:Caracter:='I';

74:Caracter:='J';

75:Caracter:='K';

76:Caracter:='L';

77:Caracter:='M';

78:Caracter:='N';

79:Caracter:='O';

80:Caracter:='P';

81:Caracter:='Q';

82:Caracter:='R';

83:Caracter:='S';

84:Caracter:='T';

85:Caracter:='U';

86:Caracter:='V';

87:Caracter:='W';

88:Caracter:='X';

89:Caracter:='Y';

90:Caracter:='Z';

97:Caracter:='a';

98:Caracter:='b';

99:Caracter:='c';

100:Caracter:='d';

101:Caracter:='e';

102:Caracter:='f';

103:Caracter:='g';

104:Caracter:='h';

105:Caracter:='i';

106:Caracter:='j';

107:Caracter:='k';

108:Caracter:='l';

109:Caracter:='m';

110:Caracter:='n';

111:Caracter:='o';

112:Caracter:='p';

113:Caracter:='q';

114:Caracter:='r';

115:Caracter:='s';

116:Caracter:='t';

117:Caracter:='u';

118:Caracter:='v';

119:Caracter:='w';

120:Caracter:='x';

121:Caracter:='y';

122:Caracter:='z';

ELSE

Caracter:='';

END_CASE;

F_ASCII_TO_STRING:=Caracter;

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**How to use it ?**--------------------------------------------------

PROGRAM PLC_PRG

VAR

ValNum:UINT;

Result:STRING;

END_VAR

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Result:=F_ASCII_TO_STRING(ValNum);

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Hoping this will meet your needs

BR