Functions Of Sm 338; Pos-Input; Encoder Value Acquisition; Gray Code/Binary Code Converter - Siemens SIMATIC S7-300 Manual

Other signal modules
7.4 Position decoder module SM 338; POS-INPUT; (6ES7338-4BC01-0AB0)
7.4.2

Functions of SM 338; POS-INPUT; encoder value acquisition

The absolute value encoder transfers its values in frames to the SM 338. The SM 338
initiates the frame transfer.
● In non-isochronous mode, the encoder values are acquired cyclically.
● In isochronous mode, the encoder values are acquired in synchronism with the
Cyclic encoder value acquisition
The SM 338 always initiates a frame transfer at the end of the programmed monoflop time.
Asynchronously to those cyclic frames, the SM 338 processes the acquired encoder values
cyclically, based on its refresh rate (see Technical data.)
Thus, cyclic acquisition returns encoder values of different ages. The difference between the
min./max. age represents the jitter (see Technical data.)
Isochronous encoder value acquisition
Isochronous encoder values acquisition is automatically set when the DP master system
operates with active constant bus cycle, and the DP slave is in synchronism with the DP
cycle.
SM 338 initiates a frame transfer in each
PROFIBUS DP cycle, at the time T
The SM 338 processes the transferred encoder values in synchronism with the PROFIBUS
DP cycle.
7.4.2.1

Gray code/binary code converter

When Gray code is set, the Gray code value returned by the absolute value encoder is
converted into binary code. When binary code is set, the values returned by the encoder
remain unchanged.
Note
When you set Gray code, the SM 338 always converts the entire encoder value (13, 21, 25
bits). As a result, any leading special bits will influence the encoder value, and the appended
bits may be corrupted.
7-12
PROFIBUS DP cycle at each T .
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S7-300 Automation System Module data
Manual, 08/2006, A5E00105505-04