Siemens SIMATIC S7-300 Manual page 237

9.6.1 Incremental encoders
Overview
Incremental encoders serve to detect position values, supplying pulses that
the FM 354 adds up to form an absolute value. After the FM 354 is switched
on, there is an offset, which cannot be determined in advance, between the
internal position value and the mechanical position of the axis. In order to
establish the position reference, the internal value must therefore be set to a
predefined value at a specific axis position. This value is stored in the ma-
chine data (MD) as a reference point coordinate (see Section 9.2.3).
Incremental The following variant applications are possible:
encoders
FM 354 Servo Drive Positioning Module
Rotary incremental encoder on linear axes
Encoders with one zero pulse per revolution may be used. The number of
encoder pulses must be a multiple of ten or a power of two.
Rotary incremental encoder on rotary axes
Encoders with one zero pulse per revolution may be used. The number of
encoder pulses must be a multiple of ten or a power of two. With indirect
encoder mounting and reference point approach with a zero pulse (MD18
< 4), you must ensure that the revolution of the rotary axis is divisible
without remainder by the cyclical zero pulse (see "Dependencies" Sec-
tions 5.3.1. and 9.5).
Linear scales on linear axes
Scales may be used with at least one reference zero pulse, or with a cyclic
zero pulse.
In comparison to rotary incremental encoders, instead of the encoder rev-
olution a period of division is used as a basis here, corresponding for ex-
ample to the segment between two zero-mark pulses.
Description of Functions
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