Lenze ELCAMZIxxx4SNNPSNN (LCU122) - Manual (Page 102)

PROFIsafe on board
Appendix
Special characteristics for the use of Lenze standard devices
6
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102 EDSLCMX3024-SPS EN 5.0
6.7 Appendix
6.7.1 Special characteristics for the use of Lenze standard devices
6.7.2 Consistent parameter data
In the PROFIBUS communication system data are permanently exchanged between the
master computer (CPU + PROFIBUS master) and the standard device via the plugged-in
slave interface module.
Both the PROFIBUS master and the CPU (central processing unit) of the master computer
access a joint memory - the dual port memory (DPM).
The DPM allows data exchange in both directions (write/read):
Щ Щ 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 a non-permissible status, the parameter data to be transmitted by must be
marked as ”consistent”.
Data communication with existing consistency
With consistency, either ”reading” or ”writing” is possible in the memory with
simultaneous access of master and CPU:
ƒ The PROFIBUS master transfers data only as a complete data record.
ƒ The CPU can only access completely updated data records.
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:
Щ Щ PROFIBUS master Dual port memory
(DPM)
Central processing
unit (CPU)
 
 CPU wants to read!
 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 is read completely by the CPU.
2. The PROFIBUS master only writes a complete record into the DPM.