Siemens Simatic Manual page 69

Fail-safe function blocks for storage and retrieval machines
Assuming there are no feedback signal errors of the brake, and the logical combination at the ON
input of the F_FDBACK is a 1 signal, then the brakes at output A20.0 ("BRAKE1") and A20.1
("BRAKE2") are opened.
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S7-Distributed Safety/SIMATIC Safety - fail-safe function blocks for storage and retrieval machines