The older E-Series need a Cat5 cable with a grounded metal shield/connector that touches something inside the MFD and confirms the connection. The switch side does not need to be metal as most switches are plastic anyway but it is still a good idea to connect the shields of different cables. You can cut and cross over the cables, you do not need a coupler. A good/cheap
installation would be one Raymarine cable, cut it in half, connect one half with the metal connector to the E120, the other half to the E80, then use any Cat5 cable to splice and cross over, going to the switch. You can use any switch or a
wifi router but a switch is simpler. Most newer switches are auto sensing, so you do not need to do the cross over.
The E90 Wide and later are auto sensing but you still need the Raymarine cable with the round connector to establish a secure connection. By the way, it is the same for
radar and sonar cables. Ethernet cables are easy to splice but you have to use the Raymarine cables on the device side.