Consistency Of Communication Blocks And Functions; Access To The Cpu Ram; Consistency Rules For Sfb 14 "Get" Or Reading Tag And Sfb 15 "Put" Or Writing Tag - Siemens SIMATIC S7-400H System Manual

S7-400H in PROFIBUS DP mode
7.2 Consistent Data
7.2.1

Consistency of communication blocks and functions

Using S7-400 the communication data is not processed in the scan cycle checkpoint; instead,
this data is processed in fixed time slices during the program cycle.
The system can always process the data formats byte, word and dword consistently, i.e. the
transfer or processing of 1 byte, 1 word = 2 bytes or 1 dword = 4 bytes can not be interrupted.
When 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, the access to this data area
can be coordinated by means of the actual "DONE" parameter, for example. Data
consistency of the communication areas transmitted locally with a communication block 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 PLC. Here, you must make allowance for the
volume of consistent data in the programming phase.
7.2.2

Access to the CPU RAM

The communication functions of the operating system access the CPU RAM in fixed block
lengths. The block length is CPU-specific. The tags for S7-400 CPUs have a length of up to
472 bytes.
This ensures that the interrupt reaction time is prolonged not due to communication load.
Since this access is performed asynchronously to the user program, you can not transmit an
unlimited number of bytes of consistent data.
The rules to ensure data consistency are described below.
7.2.3 Consistency rules for SFB 14 "GET" or reading tag and SFB 15 "PUT" or
writing tag
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 you initiate a send operation (positive edge at REQ), the system copies the data of the
send data areas SD_i to be transferred from the user program. You can write new data to
these areas after the block call, without any risk of corrupting the current send data.
Note
Completion of transfer
The transfer operation is not completed until the status parameter DONE assumes the
value 1.
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System Manual, 09/2007, A5E00267695-03
S7-400H