ETC Link must be terminated on both ends of the line.
On the non-Sensor Rack end of the line, termination is accomplished with the use of an ETC Link Terminator (aka: Ice Cube). The terminator should be connected between pins 1 and 2 in the header that plugs into the back of the 6-Pin XLR connector. (In the last few years that ETC Link was used in systems, the Ice Cube was replace by a small circuit board inside a small black plastic box)
In an ETC Sensor (Classic) Dimming Rack, this is done by: End-Of-Line DMX512 and ETC Link Termination on Sensor CEM