OPC server showing cross in OPC server connection for Alarm and Events
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Check/verify number of AE connections - it is limited as per setting on CPU parameters. PP800 or old subscriptions may linger and block the AC 800 OPC server from connecting. A Controller Reset is required to clear obsolete subscriptions.
Check/verify Product Alert for MMS AE connection on secondary network (access to ABB Library required).
Check SystemDiagnostics faceplate/interaction window, Advanced > Alarm & Event > Subscribers in Program3.Slow. Subscription status should be 1 and all three buffer counters should be zero.
Ultimately, contact your regional ABB support center to obtain professional assistance. Attach logs from a run of the AC800Fingerprint.exe tool which is located on the USB media but also on disk in recent versions of 800xA.