Hi Nick, I understand your rationale for using the 680R's on A,B as you show, but that's not quite the same termination
as the d/s for the LTC1480 shows. I would think the 120R would quench the noise even on a floating line.
On the page I referenced I also have this statement:
If the transceivers are not at the ends of the cable termination resistors are probably necessary. Something like 120 ohms, connected between the A and B cables, at each end only, stop the signal reflecting back along the cable.