Dell Intel PRO Family of Adapters - Manual (Page 114)

The Intel® Ethernet Server Adapter X520-T2 has the following indicator lights:
Label Indication Meaning
ACT Green flashing Data activity
Off No link
LNK Green on 10 Gbps link
Yellow on 1 Gbps link
Off No link
The Intel® Ethernet 10G 2P X520 Adapters have the following indicator lights:
Label Indication Meaning
LNK (green/yellow) Green on Operating at maximum port speed.
Yellow on Operating at lower port speed.
Off No link.
ACT (green) Green flashing Data activity.
Off No activity.