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Overview of the DP Base Interface

DP Base Programming Interface for CP 5613/CP 5614

C79000-G8976-C108-01

3.3 Consistent Access to the process image

Conflicts Accessing the Process Image

If, for example, your user program is currently reading the data of the DP slave

from the process image and at exactly the same time the DP master overwrites this

data with new data, your program could read the first few bytes from the previous

DP cycle and the remaining bytes from the current cycle. The data would then be

corrupted and inconsistent.

The Read Consistency Option

On the CP 5613 and CP 5614, you can decide whether or not you want to read the

data of the process image of the input data or diagnostic data consistently. In some

situations, for example, when the data is only 2 bytes long, you do not need to

select consistency since inconsistency can only occur when the data is longer.

Consistency can be guaranteed up to a maximum data length of 244 bytes.

Writing Data is always Consistent

On the CP 5613 and CP 5614, output data are always written consistently due to

the transfer mechanism on the CP 5613/CP 5614 up to the maximum data length

of 244 bytes.

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Siemens SIMATIC NET PROFIBUS Manual de servicio Pagina 42