Siemens SIMATIC S5 Manual page 96

Application Examples IP 267
6.5 Reference Point Approach with Limit Switch
This method of determining the reference point also consists of three approach
sections ( 5.4). A separate reference point switch is not required. The REF input
on the terminal block remains unassigned and the RPA reference bit in the
positioning message frame is not set. The path of the second approach section
(switch overtravel) should be long enough to enable the limit switch contacts
again.
You can execute a reference point approach with limit switch I+ using FB 54. A
separate reference point switch is not required in this example. The positioning
job data is written direct into the PIQ. The program is started by a positive edge at
input I 0.3 (see OB 1).
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EWA 4NEB 812 6061-02a