Full duplex means four wires, not two
Unlike the half-duplex part (which shares the same 8-DIP footprint and 5V supply), the MAX490EPA+ supports full-duplex communication using separate transmit and receive pairs. That gives you simultaneous bidirectional data flow — useful for poll-response protocols where the master doesn't want to wait for bus turn-around. The trade-off is wiring: you need four conductors (plus ground) instead of two. If your application is half-duplex multi-drop, the half-duplex part is the simpler fit; if you need continuous two-way streaming over RS-422 or RS-485, this part is the one.