SIMATIC S7-SCL for S7-300 and S7-400 manual

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The simple expression stands for the maximum number of characters in the
STRING. All the characters of the ASCII code are permitted in a character string. A
string can also include special characters, for example, control characters and
nonprintable characters. You can enter these using the syntax $hh, where hh
stands for the value of the ASCII character expressed in hexadecimal (example:
'$0D$0AText' )
When you declare the memory space for character strings, you can define the
maximum number of characters that can be stored in the string. If you do not
specify a maximum length, a string with a length of 254 is created.