Siemens SIMATIC User Manual

Other signal modules
7.4 Position decoder module SM 338; POS-INPUT; (6ES7338-4BC01-0AB0)
S7-300 Module data
502 Manual, 06/2017, A5E00105505-AJ
7.4.3 Functions of SM 338; POS-INPUT; encoder value acquisition
7.4.3.1 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
PROFIBUS DP cycle at each T
i
.
Cyclic encoder value acquisition
The SM 338 always initiates a frame transfer at the end of the programmed monoflop time.
Asynchronously to these cyclic frames, the SM 338 processes the acquired encoder values
cyclically, based on its refresh rate (refer to the chapter "Technical data of SM 338; POS-
INPUT (Page 512)").
Thus, cyclic acquisition returns encoder values of different ages. The difference between the
min./max. age represents the jitter (refer to the chapter "Technical data of SM 338; POS-
INPUT (Page 512)".)
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
i
.
The SM 338 processes the transferred encoder values in synchronism with the PROFIBUS
DP cycle.
7.4.3.2 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.

Siemens SIMATIC Specifications

General

Brand	Siemens
Model	SIMATIC
Category	Control Unit
Language	English