Siemens SIMATIC S7-1500 Function Manual page 34

System-specific maximum data consistency for S7-1500:
No inconsistency occurs if the system-specific maximum size of the consistent data is kept
to. With an S7-1500, communication data is copied consistently into or out of the user
memory in blocks of up to 512 bytes during the program cycle. Data consistency is not
ensured for larger data areas. Where defined data consistency is required, the length of
communication data in the user program of the CPU must not exceed 512 bytes. You can
then access these data areas consistently, for example from an HMI device by
reading/writing tags.
If more data than the system-specific maximum size needs to be transferred consistently,
you yourself must ensure the data consistency with suitable measures in the user program.
Ensuring data consistency
Use of instructions for access to common data:
If the user program contains instructions for communication that access common data, for
example TSEND/TRCV, you can coordinate access to this data area yourself, for example
using the "DONE" parameter. The data consistency of the data areas that are transferred
with an instruction for communication can therefore be ensured in the user program.
Note
Measures in the user program
To achieve data consistency, you can copy transferred data to a separate data area (for
example, global data block). While the user program continues to work with the original data,
you can transfer the data saved in the separate data area consistently with the
communication instruction.
For the copying, use uninterruptible instructions such as UMOVE_BLK or UFILL_BLK. These
instructions ensure data consistency up to 16 KB.
Use of PUT/GET instructions or Write/Read via HMI communication:
In S7 communication with the PUT/GET instructions or Write/Read via HMI communication,
you need to take into account the size of the consistent data areas during programming or
configuration. In the user program of an S7-1500 as server, there is no instruction available
that can coordinate the data transfer in the user program. The data exchanged using
PUT/GET instructions updates the S7-1500 while the user program is running. There is no
point in time within the processing of the cyclic user program at which the data is exchanged
consistently. The length of the data area to be transferred should be smaller than 512 bytes.
Additional information
● You will also find the maximum amount of consistent data in the device manuals of the
communications modules in the Technical Specifications.
● You will find further information on data consistency in the description of the instructions
in the STEP 7 online help.
Communication
Function Manual, 11/2019, A5E03735815-AH
Communications services
3.5 Data consistency
33