Consistent Data; Consistency Of Communication Blocks And Functions; Consistency Rules For Sfb 14 "Get" Or Read Variable, And Sfb 15 "Put" Or Write Variable - Siemens SIMATIC CPU 410 System Manual

Supplementary information

18.12 Consistent data

PC/PG as Engineering System (ES)
To be able to use a PC as Engineering System, you need to configure it under its name as a
PC station in HW Config. The ES is assigned to a CPU and is capable of executing STEP 7
functions on that CPU.
If this CPU fails, communication between the ES and the fault-tolerant system is no longer
possible.
18.12 Consistent data
18.12.1

Consistency of communication blocks and functions

On the S7-400H, communication jobs are not processed in the cycle control point but rather
in fixed time slices during the program cycle.
The byte, word and double word data formats can always be processed consistently in the
system, in other words, the transmission or processing of 1 byte, 1 word = 2 bytes or 1
double word = 4 bytes cannot be interrupted.
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.
18.12.2 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:
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CPU 410 Process Automation/CPU 410 SMART
System Manual, 05/2017, A5E31622160-AC