Samsung ML-2151N-XBH - Manual (Page 215)

P ROGRAMME I NSTALLING P RINTER O PTIONS F.8
If you are replacing the network interface card with new
one or the serial interface card, remove the two screws and
the old card.
5 Align the connector on the network or serial interface card
with the connector on the control board. Push the card
firmly into the connector until it is completely inserted into
place.
6 Tighten the two screws which you removed in Step 4.