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Product Information on the Reference Manual Programmable Logic Controllers S7-300 Module Data Release 3
A5E00409892-01
1.1.2
Functions of the SM 338; POS INPUT
1.1.2.1
Encoder value acquisition
The absolute value encoder transfers its encoder values in messages to the
SM 338. The transfer of the message is initiated by the SM 338.
•
In non-synchronous operation, the encoder values are acquired while it is free
running.
•
In synchronous operation the encoder values are acquired synchronized to the
PROFIBUS DP cycle at each T
i
.
Free running encoder value acquisition
The SM 338 always initiates the transfer of a message after the end of the
parameterized monoflop time.
Asynchronous to these free running messages, the SM 338 processes the
acquired encoder values during the cycle of its updating rate (see Technical Data).
Thus, in the case of free running encoder value acquisition, encoder values of
different ages result. The difference between the maximum and minimum age is
the jitter (see Technical Data).
Synchronous encoder values acquisition
Synchronous encoder values acquisition is automatically set if, in the DP master
system, the equidistant bus cycle is activated and the DP Slave is synchronized to
the DP cycle.
The SM 338 initiates the transfer of a message in each PROFIBUS DP cycle
at the time T
i
.
Synchronous to the PROFIBUS DP cycle, the SM 338 processes the transferred
encoder values.
