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If the user program calls communication blocks, such as SFB 12 "BSEND" and SFB 13
"BRCV", which are only used in pairs and access shared data, access to this data area can
be coordinated by the user by means of the "DONE" parameter, for example. The
consistency of data transmitted locally with these communication blocks can thus be ensured
in the user program.
In contrast, S7 communication functions do not require a block such as SFB 14 "GET", SFB
15 "PUT", in the user program of the target device. Here, you must make allowance for the
volume of consistent data in the programming phase.
Access to work memory of the CPU
The communication functions of the operating system access the CPU's work memory in
fixed block lengths. Blocks for S7-400H CPUs have a variable length of up to 472 bytes.
This ensures that the interrupt response time is not prolonged due to communication load.
Because this access is performed asynchronously to the user program, you cannot transmit
an unlimited number of bytes of consistent data.
The rules to ensure data consistency are described below.
16.11.2
Consistency rules for SFB 14 "GET" or read variable, and SFB 15 "PUT" or
write variable
SFB 14
The data are received consistently if you observe the following points:
Evaluate the entire, currently used part of the receive area RD_i before you activate a new
request.
SFB 15
When a send operation is initiated (rising edge at REQ), the data to be sent from the send
areas SD_i are copied from the user program. You can write new data to these areas after
the block call command without corrupting the current send data.
Note
Completion of transfer
The send operation is not completed until the status parameter DONE assumes value 1.
CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 10/2013, A5E32631667-AA
Supplementary information
16.11 Consistent data
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