Siemens SIMATIC S7 F Manual page 193

Fail-Safe Blocks
Signal State 1 or 0
Signal state 1 at the block I/O of the data type BOOL always means that the event
described (e.g. error on channel x) is active.
Making Block I/Os Visible
Proceed as follows:
1. Double-click the block's header.
2. Select the "Inputs/Outputs" tab in the "Properties" dialog box.
3. Scroll to the right until the "Invisible" column appears.
4. Right-click the "Invisible" selection cross of the block I/O.
Result: The invisible block I/O becomes visible in CFC.
Fail-Safe Systems
A5E00085588-03
8-5