Detailed Description
2.12 Block descriptions
PLC Basic Program Solution Line (P3 sl)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0
2-231
Strings
General
The STRING data type is required by certain services of the basic program. For this reason,
some additional facts about the string structure and general handling procedures for
parameter assignments are given below.
Structure of STRING
A data of type STRING is generally stored (defined) in a data block. There are two methods
of defining a string:
1. Only the data type STRING is assigned to a variable. The STEP7 compiler automatically
generates a length of 254 characters.
2. Data type STRING is assigned to a variable together with a string length in square
parenthesis (e.g., [32]). With this method, the STEP7 compiler generates a string length
corresponding to the input.
Two bytes more than prescribed by the definition are always stored for variables of the
STRING data type. The STEP7 compiler stores the maximum possible number of characters
in the 1st byte. The 2nd byte contains the number of characters actually used. Normally, the
useful length of the assigned string is stored in byte 2 by the compiler. The characters
(1 byte per character) are then stored from the 3rd byte onwards.
String parameters are generally assigned to blocks of the basic program by means of a
POINTER or ANY. Such assignments must generally by made using symbolic programming
methods. The data block, which contains the parameterizing string, must be stored in the
symbol list. The assignment to the basic program block is then made by means of the
symbolic data block name followed by a full stop and the symbolic name of the string
variable.
Determining offset addresses for data block structures
General
Another task, which occurs frequently, is symbolic determination of an offset address within
a structured DB, e.g., an ARRAY or STRUCTURE is stored somewhere within the DB. After
loading the address register symbolically with the start address, you might like to access the
individual elements of the ARRAY or STRUCTURE via an address register. One way of
loading the address register symbolically is to use an FC whose input parameter is a pointer.
The address of the ARRAY or STRUCTURE is then assigned symbolically to the input
parameter of this FC in the program. The program code in the FC now determines the offset
address from the input parameter, and passes the offset address in the address register
(AR1) to the calling function. Symbolic addressing is thus possible even with indirect access.
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