sometimes IRT communication not possible when directly connected to ERTEC4000 based Scalance switch

When IRT communication is used, sometimes IRT communication someteimes can not be established in the following scenario:

• netX is connected to e.g. Scalance X204IRT switch
• netX and Scalance switch are powered on at the same time
• PLC (acting as SyncMaster) is connected to Scalance 
• Scalance switch has a new Scalance firmware V5.6.0
  • older Scalance firmwares e.g. 5.3.0 do not lead to issue

In this setup, sometimes IRT communication between netX and PLC can not be established. netX claims to be synchronized (as seen in RTC3PSM), has CLRPC based connection establishment fully done and even sends out cyclic RTC3 frames.

However, the RTC3 frames are sent at wrong point in time and use a wrong cycle counter. The netX is not correctly synchronized in this case.

When analyzing network capture of failed IRT communication, the root cause seems to be one set of wrong/unexpected RTSyncFU and FollowUp PDUs generated by the switch. If these are not visible, IRT communication can be established in same scenario with same devices.