Lenze E84Dxxxx - Manual (Page 62)

EDS84DPBUS EN 2.0 - 09/2010
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62
Communication manual 8400 protec PROFIBUS
Parameter data transfer
Consistent parameter data
8.4 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 permits a data exchange in both directions (write/read):
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 status, 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:
 The PROFIBUS master transfers data only as a complete data set.
 The CPU can only access completely updated data sets.
 The PROFIBUS master cannot read or write data as long as the CPU accesses consistent
data.
The result becomes clear from the example below:
1. As the master can only write if the CPU does not read, the master waits until the data
are read completely by the CPU.
2. The master only writes a complete data set into DPM.
Configuring consistent data
Central processing unit
(CPU)
Dual port memory
(DPM)
PROFIBUS master
CPU wants to read! Master wants to write simultaneously!
Central processing unit
(CPU)
Dual port memory
(DPM)
PROFIBUS master

Note!
Consistency is achieved by an appropriate PROFIBUS master configuration.
Please refer to the corresponding documentation for your configuring software
for this purpose.