Siemens SIMATIC S7-300 Series Reference Manual page 33

CPU 312 IFM to 316-2 DP
PUT/GET functions of S7 communication, or
reading/writing variables via OP
communication, are processed during the
cycle checkpoint of the CPU.
A defined process interrupt reaction time is
ensured by consistent copying of
communication variables in blocks of 32 bytes
(CPU Versions lower than described in this
manual: Blocks of up to 8 Bytes) into/out of
user memory during the cycle checkpoint of
the operating system. Data consistency is not
guaranteed for any larger data areas.
Therefore, communication variables in the
user program must not exceed a length of 8
or 32 byte if data consistency is required.
If you copy communication variables using
SFC 81 "UBLKMOV", the copying process is
not interrupted by higher priority classes.
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01
CPU 318-2
PUT/GET functions of S7 communication, or
reading/writing to variable via OP
communication are processed in defined time
windows in the CPU 318-2 operating system.
For that reason, the user program can be
interrupted after every command
(Byte/Word/Double Word command) when a
communication variable is being accessed.
The data consistency of a communication The data consistency of a communication
variable is therefore only possible within the
limits of the command boundaries used in the
user program.
If a data consistency size greater than Byte,
Word or DWord is required, communication
variables in the user program must always be
copied using SFC81 "UBLKMOV" that
guarantees consistent reading/writing of the
complete communication variable area.
CPUs
1-17