Wiring Mesa 7i95t inputs in parallel with DRO

Hi,
Thanks to much help my motion system is now moving about nicely so I have moved on and hit a new problems and google isn't turning up any previous work on the subject to plagerise.

I have a DRO (magnetic) and I would like to keep it but also put the encoders into LinuxCNC. (have your cake and eat it so to speak)
However being a bit of an electronic engineer (retired industrial physicist) I worry about loads and impedences.
The 7i95t manual section on the high speed encoders ofers me:
   DIFFERENTIAL MODE IMPEDANCE the min=118 max=122 Ohms
and I'm perplexed. I suspect I am thinking of something different.
I guess I could put a unity gain buffer amplifier in each pair but that seems OTT.
Do Mesa do schematics? I can't find any. Anybody done this? Pointers? ELI75

And also thinking of those encoder ports...
I am using Axis5 to do PWM to control the spindle but I'd like to read spindle speed back to the control too.
If I put a second sensor on the existing chopper disk that drives the front panel display can I wire that into the encoder and somehow get something out I can crunch into rpm? (I have discovered writing your own HAL things in C so I'm a happy bunny there).
The port says 3MHz rate and I'd only need about 5Kpps pulses counted. Again I could fake up a B pulse by just delaying sensor out but again a lot of bother for something that can probably be configured round.

I like that. Great idea. I have no idea if I can so I see the DRO coming to bits shortly.

Impedances? The most horrible thing I have had to do recently was to try to explain the Smith Chart display on a cheap VNA (aka  network analyser) to an older radio ham who could just about manage that 50 and 75 are different. It scanned the range of frequencies you gave it as about twenty discrete points and then interpolated a line in between. Great for almost all real problems on a 3" display but if you set in to scan from DC to the speed of light on a piece of wire hanging down the garden you get a bunch of virtually random points with straight lines criss-crossing the display. Very pretty but...

That said I use a nanoVNA to match my piece of wire hanging down the garden.

Whether you can wire your DRO in parallel with the 7I95T encoder inputs depends on 2 things:

Do the encoders have differential outputs?

and if they are differential, are the DRO inputs terminated?

The 7I95T encoder inputs (when in differential mode) are terminated (120 Ohms nominal)
so if the DRO is terminated also this may be a pretty heavy load on the encoders.
If this is the case one option would be to buy an encoder splitter for each axis, like Mesa's ENCY
or make your own with a couple RS-422 driver/receiver IC's

On the encoder, you can use a single "A" input for counting frequency/RPM, you just need to set the
encoder into "counter" mode in the hal file