Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 44)

Communication
3.2 Communication services
CPU 31xC and CPU 31x, Technical data
3-16
Manual, Edition 08/2004, A5E00105475-05
3.2.9 Data consistency
Properties
A data area is considered consistent, if the operating system can read/write access the data
area in a continuous block. Data exchanged collectively between the stations should belong
together and originate from a single processing cycle, that is, be consistent. If the user
program contains a programmed communication function, for example, access to shared
data with X­SEND/ X­RCV, access to that data area can be coordinated by means of the
"BUSY" parameter itself.
With PUT/GET functions
For S7 communication functions, such as PUT/GET or write / read via OP communication,
which do not require a block in the user program on the CPU (operating in server mode),
allowances must be made in the program for the extent of the data consistency. The
PUT/GET functions for S7 communication, or for reading/writing variables via OP
communication, are executed at the CPU's scan cycle checkpoint. In order to ensure a
defined process interrupt reaction time, the communication variables are copied in consistent
blocks with a maximum length of 64 bytes (CPU 317: 160 bytes) to / from work memory at
the scan cycle checkpoint of the operating system. Data consistency is not guaranteed for
larger data areas.
Note
Where defined data consistency is required, the length of communication variables in the
CPU's user program may not exceed 64 bytes (CPU 317: 160 bytes.)
3.2.10 Communication via PROFINET (only CPU 31x-2 PN/DP)
What is PROFINET??
Within the framework of Totally Integrated Automation (TIA), PROFINET represents a
consequent enhancement of:
•
PROFIBUS DP, the proven field bus, and
• Industrial Ethernet, the communication bus at cell level.
Experience gained from both systems was and is being integrated into PROFINET.
PROFINET is an Ethernet-based automation standard of PROFIBUS International
(previously PROFIBUS Users Organization e.V.), and defines a multi-vendor communication,
automation, and engineering model.