Product Overview - Siemens SIMATIC S7 Equipment Manual

Product Overview

Position encoding
with ultrasonic
sensors
Operating modes
SM 338 Equipment Manual
(4) J31069-D401-U1-A0-7618
The SM 338 is a position encoding module for the S7-300 programmable
controller. The module is connected to the P bus of the programmable con-
troller, and its parameters are assigned by the CPU. The SM 338 belongs to
the family of signal modules.
Position encoding is performed by ultrasonic sensors. These sensors offer the
following primary advantages.
No wear and tear
S
High protection class
S
Steady accuracy regardless of the sensor length
S
Low costs for longer sensors (> 500 mm)
S
Up to four measuring points (magnets) per sensor possible
S
The digital sensor interface (RS 422) is not sensitive to interference and
S
permits lines up to 50 m in length.
The SM 338 does not condition the data. Its only function is to supply the
CPU with counting values. Eight counters are available for the acquisition of
measured values.
Up to four sensors can be connected on the SM 338 module, and each sensor
can contain up to four measuring points. The sum of the measuring points on
all connected sensors may not exceed 8.
The sensors connected to the module can operate in synchronous or asyn-
chronous mode. This requires that the parameters of the selected sensors be
assigned appropriately.
Synchronous mode
The selected sensors operate synchronously (i.e., at the end of the cycle time,
the counting values of all synchronous sensors are transferred to the input
area of the dual port RAM). If enabled, an end-of-cycle alarm to the CPU is
then triggered. Synchronous mode does not use a parameterized, sensor-re-
lated, measuring cycle time.
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