Lenze E82ZAFPC010 - Manual (Page 87)

Appendix
Consistent parameter data
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87 EDS82ZAFPC010 EN 4.0
11.2 Consistent parameter data
In the PROFIBUS communication system, data are permanently exchanged between the
host (CPU + PROFIBUS master) and the standard device via the plugged−on slave interface
module.
Both the PROFIBUS master and the CPU (central processing unit) of the host access a joint
memory − the dual port memory (DPM).
The DPM allows data exchange in both directions (write/read):
у у
PROFIBUS master Dual port memory
(DPM) Central processing
unit (CPU)
It could happen that a slower PROFIBUS master writing would be overtaken by a faster CPU
reading within a cycle time without any further data organisation.
To avoid such an impermissible state, the parameter data to be transmitted must be
marked as "consistent".
Data communication with existing consistency
With consistency, either "reading" or "writing" is possible when the master and the CPU
simultaneously access the memory:
ƒ The PROFIBUS master transfers data only as a complete data set.
ƒ The CPU can only access completely updated data sets.
ƒ The PROFIBUS master cannot read or write data as long as the CPU accesses
consistent data.
The result becomes clear from the example below:
у у
PROFIBUS master Dual port memory
(DPM) Central processing
unit (CPU)
0 1
0 CPU wants to read!
1 PROFIBUS master wants to write simultaneously!
1. As the PROFIBUS master can only write if the CPU does not read, the master has to wait
until the data are read completely by the CPU.
2. The PROFIBUS master only writes a complete data set into the DPM.