6.4.5 String comparison
Operand
Input / output
Data type
IN1
VB, Constant string, LB, *VD, *LD, *AC
String
IN2
VB, LB, *VD, *LD, *AC
String
Comparison string instructions are used for comparing
two ASCII strings: IN1=IN2 ， IN1<>IN2
In LAD, when the comparison result is true, the
comparison contact will be turned on.
The maximum length of a single constant string is 126
bytes.The maximum combined length of the two constant
string is 242 bytes.
Attention:The following conditions are serious errors .
These errors will cause the PLC to immediately stop the execution of the program:
1.Enter illegal indirect address.
2.Enter a string of more than 254 characters in length.
3.The start address and length of the string cannot be put into a specified memory
area.
ASCII constant string data type format:
String is a series of characters and the corresponding memory address, each
character is stored in a byte.The value of the first byte of a string is the length of the
string.If a constant string is entered directly into the program editor or data block,
the string must start and end with double quotation marks ("string constant").
The following memory map shows the format of the string data type.The length of a
string can be between 0 and 254 characters.The maximum length of the string is 255
bytes.
String
Character
Character
Character
Character
Character
Character
length
..................................
1
2
5
254
3
4
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 254
90
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