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Parameter data transfer

Consistent parameter data

110

EDSMF2133IB EN 5.0

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 allows data exchange in both directions (write/read):

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:

PROFIBUS master

Dual port memory

(DPM)

Central processing

unit (CPU)

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.

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