Siemens SIMATIC S7-400H - Consistency Rules for SFB 15 “PUT” or Write Tag; Data Transmission Without Data Consistency

Installation of a CPU 41x-H
4-44
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Consistency Rules for SFB 15 “PUT” or Write Tag
Using SFB 15 “PUT” data are transmitted consistently if you adhere to the
following consistency rules:
• Active CPU (data sender): Write the receive area in the OB by calling SFB 15
or – if this is not possible – write the send area when the processing of SFB 15
is completed.
• Active CPU (data sender): Write only as much data to the send area as the
block size of the passive CPU (data receiver) specifies.
• Passive CPU (data receiver): Read the received data from the receive 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: 64 bytes are
sent although the block size of the passive CPU (data receiver) is only 32 bytes.
Data transmission
User data
16 Byte
User data
Send
Range
Receive
Range
Active CPU with SFB PUT or write tag
(data sender)
(data receiver)
Passive CPU
32 bytes
32 bytes
16 Byte
16 Byte
16 Byte
1
2
3
4
1234
1234
Figure 4-10 Data transmission without data consistency
Consistent transfer of larger data blocks spanning several tags can be ensured in
the user program of the S7-400 using SFC 81 “UBLKMOV” (uninteruptable block
move).
These data can be consistently accessed using SFB 14 “GET”, SFB 15 “PUT or
reading/writing the tag, for example.