Fail-safe function blocks for storage and retrieval machinesS7-Distributed Safety/SIMATIC Safety - fail-safe function blocks for storage and retrieval machines- 3-53 -Assuming there are no feedback signal errors of the brake, and the logical combination at the ONinput of the F_FDBACK is a 1 signal, then the brakes at output A20.0 ("BRAKE1") and A20.1("BRAKE2") are opened.