Consistency Rules For Sfb 14 “Get” And Reading Tags; Data Transmission Without Data Consistency - Siemens SIMATIC S7-400H Manual

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.
Active CPU with SFB GET or read tag
(data receiver)
32 bytes
Receive
Range
32 bytes
Figure 4-9

Data transmission without data consistency

Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
1 2 3 4
User data
Data transmission
Installation of a CPU 41x-H
Passive CPU
(data sender)
Byte 8
1
Byte 8
2
Byte 8
3
Byte 8
4
1 2 3 4
User data
Send
Range
4-43