Lenze 931K - Manual (Page 18)

PROFIBUS communication
Consistent parameter data
Structure of the receive and response message with PKW mechanism
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18 KHB 13.0004−EN 2.1
5.2 Consistent parameter data
In the PROFIBUS communication system, a permanent data exchange takes place between
the master computer (CPU + PROFIBUS master) and the slave.
The PROFIBUS master and the CPU (central processing unit) of the master computer access
a common storage medium for this purpose − the dual port memory (DPM).
The DPM enables a data exchange into both directions (read/write):
у у
PROFIBUS Master Dual Port Memory
(DPM) Central Processing
Unit (CPU)
Without any further data organisation, a slower write action of the PROFIBUS master
might be overtaken by a faster read action of the CPU within a cycle.
To prevent this from happening, the parameter data to be transferred must be marked as
"consistent".
Data communication with existing consistency
With consistency either "read" or "write" is possible in the data memory if master and CPU
have access at the same time:
ƒ The PROFIBUS master only transfers the data 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 is shown in the following example:
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 at the same time!
1. Due to the fact that the PROFIBUS master can only write if the CPU does not read, the
PROFIBUS master has to wait until the CPU has read all of the data.
2. The PROFIBUS master only writes complete data records into the DPM.
Configuration of consistent data
Consistency is achieved by configuring the PROFIBUS master accordingly. Use the
corresponding instructions of your configuration software for this purpose.