Can the Ethernet 1000BASE-X PCS/PMA core be used to send frames in only one direction? That is, can you send frames out of the transmitter of one core (with the receiver of that core disabled) to the receiver of another core (with that core's transmitter disabled).
The transmit and receive engines of the Gigabit Ethernet MAC core are independent but they are not in the 1000BASE-X PCS/PMA core. With the 1000BASE-X PCS/PMA core, there is feedback from the receiver to the transmitter that is used to indicate when the link is up. Therefore, if the receive path does not exist, initialization data will not be seen by the receiver and the link will never come up.
However, you can work around to "trick" the transmitter into thinking there is a receiver attached without having to physically attach it to the other core. To work around, use serial loopback. Serial loopback not only loops back the transmitted data to the receiver but also send data out of the TXP/TXN pins of the MGT. Therefore, frames will be transmitted to the receiving device as intended, but initialization data will also be looped back to the receiver, which will allow the link to come up.