Name: Siemens SIMATIC S7-300 Module data
Brand: Siemens
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Special Signal modules

5-10

Programmable Logic Controllers S7-300 Module Data

A5E00105505-03

5.4.3 Functions of the SM 338; POS INPUT

5.4.4 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

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

Synchronous to the PROFIBUS DP cycle, the SM 338 processes the transferred

encoder values.

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