Finisar’s FCMJ-8520/8521-3 1000BASE-T Copper Small Form Pluggable (SFP)
transceivers are based on the SFP Multi Source Agreement (MSA)1. They are compatible
with the Gigabit Ethernet and 1000BASE-T standards as specified in IEEE Std 802.32.
The 1000BASE-T physical layer IC (PHY) can be accessed via I2C, allowing access to
all PHY settings and features.

The FCMJ-8520-3 uses the SFP’s RX_LOS pin for link indication, and 1000BASE-X
auto-negotiation should be disabled on the host system. The FCMJ-8521-3 is compatible
with 1000BASE-X auto-negotiation, but does not have a link indication feature
(RX_LOS is internally grounded). See AN-2036, “Frequently Asked Questions
Regarding Finisar’s 1000BASE-T SFPs”, for a more complete explanation on the
differences between the two models and details on applications issues for the products.