In SERCOS phase 4, the behavior when the Fiber Optics Cables were removed was not consistent with the behavior of powing cycling a SERCOS drive. Both conditions effectively block communication with the drive so consistency is to be expected.

Disconnecting the fiber cables caused the XL controller to reset back to SERCOS Phase 0. Power cycling resulted in the controller remaining in phase 4 with no diagnostic information that anything was wrong.

On a power cycle, the SERCOS code was getting stuck in the PHASE4_C1D_ERROR state. Using the Flag that I added for Simple Sercos to detect a drive power cycle I was able to force the controller back to phase 0 so that it was consistent with a fiber break.

In both cases the controller will bring the drives back up to phase 4 once communication is re-established.