Siemens SIMATIC NET PROFIBUS Manual de servicio descargar pdf (Pagina 25)

Overview of PROFIBUS DP

DP Base Programming Interface for CP 5613/CP 5614

C79000-G8976-C108-01

2.7 Separation of the Slave Data from the User Program

The process image is separate from the user program and the slaves.

To increase efficiency, the DP standard does not include flow control. The timing of

the cyclic updating of the process image of the DP master implementation is

connected neither to the DP slaves nor to the user program. Examples of different

situations are shown below:

Example: The slave writes too quickly

If, for example, an analog slave modifies its output data quickly (sequence 1.11,

1.2, 1.3, 1.42, 1.5, 1.6, 1.7, 1.8, ...), the DP master only receives a sequence of

"snapshots" that it enters in the process image (sequence 1.11, 1.3, 1.5, 1.8, ...).

Example: The user program reads too quickly

If the user program polls the process image extremely quickly, it obtains the values

more than once since it overtakes the polling cycle of the master. Based on the

example above, the user program then reads a sequence 1.1, 1.1, 1.1, 1.1, 1.3,

1.3, 1.3, 1.5, 1.5, 1.5, ...

Example: The user program writes too quickly

If the user program modifies the process image extremely quickly (sequence 1, 2,

3, 4, 5, 6, ...), it overtakes the polling cycle of the DP master. This then only

transfers "snapshots" to the slave (sequence 1, 4, 6, ...).

Example: The user program reads too slowly

If the user program only polls the process image occasionally, for example,

because it has various tasks to execute in the meantime, it is possible that some

values will be skipped. The sequence 1.1, 1.2, 1.3, 1.4 in the process image then

becomes, for example 1.1, 1.3 etc. in the user program.

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Siemens SIMATIC NET PROFIBUS Manual de servicio Pagina 25