Why NVLs are not working in Ethernet port 2 of my TM251?

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NVLs (global network variables) will work in the Ethernet port 2 of the M251 only if the Ethernet port 1 status is "Data Exchange".
Ethernet port 1 is in "Data Exchange" when the ethernet cable is connected and the PLC has a valid IP address.

Note: You can check the status of the ETH port 1 using the system variable PLC_GVL.ETH_R.i_wPortALinkStatus.