BECKHOFF CX51x0 - Manual (Page 74)

Commissioning/Configuration
The interruption is indicated by "LNK_MIS B" and "LNK_MIS A".
The next example shows a failure of the "return line":
In this case the second cable is faulty. The terminals at the coupler continue to run without malfunction. The
System Manager indicates the behavior as follows:
CX51x0 74 Version 1.0