Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked
at it, I like the idea how D1 and D2 are used to terminate through the
voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then
reverse D1 and D2 to no termination. Very clever. I ran this circuit
on the simulator and toggle J1 between 5v and 0v. Unfortunately, I
saw no change on the right side of R11. Checking on the circuit I
notice that Q5 and Q1 can never turn on regardless if the right side
of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets
forward bais to turn the transistor. The same is Q5. I can't seem to
see any bipolar configuration for Q2 to switch Q5 and Q1 on or off.
Can you see if I redrew your schematic correct. I hope to get your
schematic to work on Workbench first before I try to bread-board it.
Your idea for the diode to switch on or off the termination make
sense. I just have to get the logic controls to work proper. Please
send me your feedback.

Caesar