Allen-Bradley ControlNet 1786-RPFS Installation Instructions Manual page 5

3. Press the module down onto the DIN rail until flush.
The locking tab should snap into position and lock the module to the
DIN rail.
4. If the module does not snap into position, use a screwdriver or similar
device to move the locking tab down while pressing the module flush
onto the DIN rail. Release the locking tab to lock the module in place.
If necessary, push up on the locking tab to lock.
ControlNet Modular Repeater Short-distance Fiber Module 5
AB PLCs