Technical data

Appendix
27.7 Accessing Process and I/O Data Areas
Programming with STEP 7
676 Manual, 05/2010, A5E02789666-01
27.7.2 Accessing the Peripheral Data Area
The peripheral data area can be divided into the following:
• User data and
• Diagnostic and parameter data.
Both areas have an input area (can only be read) and an output area (can only be written).
User Data
User data is addressed with the byte address (for digital signal modules) or the word address (for
analog signal modules) of the input or output area. User data can be accessed with load and
transfer commands, communication functions (operator interface access), or by transferring the
process image. User data can be any of the following:
• Digital and analog input/output signals from signal modules
• Control and status information from function modules
• Information for point-to-point and bus connections from communication modules (only S7-300)
When transferring user data, a consistency of a maximum of 4 bytes can be achieved (with the
exception of DP standard slaves, see Setting the Operating Behavior). If you use the "transfer
double word" statement, four contiguous and unmodified (consistent) bytes are transferred. If you
use four separate "transfer input byte" statements, a hardware interrupt OB could be inserted
between the statements and transfer data to the same address so that the content of the original 4
bytes is changed before they were all transferred.
Diagnostic and Parameter Data
The diagnostic and parameter data of a module cannot be addressed individually but are always
transferred in the form of complete data records. This means that consistent diagnostic and
parameter data are always transferred.
The diagnostic and parameter data is accessed using the start address of the module and the data
record number. Data records are divided into input and output data records. Input data records can
only be read, output data records can only be written. You can access data records using system
functions or communication functions (user interface). The following table shows the relationship
between data records and diagnostic and parameter data.
Data Description
Diagnostic data If the modules are capable of diagnostics, you obtain the diagnostic data of the module
by reading data records 0 and 1.
Parameter data If the modules are configurable, you transfer the parameters to the module by writing
data records 0 and 1.