Siemens SIMATIC S7-400H - Consistency Rules for SFB 14 “GET” and Reading Tags; Data Transmission Without Data Consistency

Installation of a CPU 41x-H
4-43
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
4.10.3 Consistency Rules for SFB 14 “GET” and Reading Tags
Using SFB 14 “GET” data are transmitted consistently if you adhere to the
following consistency rules:
• Active CPU (data receiver): Read the receive area in the OB by calling SFB 14
or – if this is not possible – read the receive area when the processing of SFB
14 is completed.
• Passive CPU (data sender): Write only as much data to the send area as the
block size of the passive CPU (data sender) specifies.
• Passive CPU (data sender): Write the data to be sent to the send area using an
interrupt block.
The following is an example of a situation in which consistent data transmission
cannot be ensured because it violates the second consistency rule: 32 bytes are
sent although the block size of the passive CPU (data sender) is only 8 bytes.
Data transmission
User data
Byte 8
Byte 8
32 bytes
User data
Send
Range
Receive
Range
(data sender)
(data receiver)
Passive CPU
Active CPU with SFB GET or read tag
32 bytes
Byte 8
Byte 8
1
2
3
4
1234
12 3 4
Figure 4-9 Data transmission without data consistency