A techincal question on the Mesa 7i76e input limits

I recently had to interface a servo drive's encoder to the Mesa 7i76E board.  I ended up using a high speed optocoupler and converting the signals to single ended inputs.  The solution skirted the core question though, which is:  Are the input voltage limits on the Mesa relative to the reference plane of the differential signal or Mesa's ground plane?  In my case, the differential signal was almost thirty volts relative to "earth" while the differential of that voltage was only +/- 5V.

The common mode range of a 7I76E encoder input is +12 and -7V
30V is way outside of the limit.

Normally the common mode should be close to +2V.

30V likely means you have a missing ground that's needed establish
the common mode

It's a low-ball Chinese drive. They made the simulated encoder output isolated and didn't add a common ground point for that circuit. All the other inputs and outputs on the drive have a common but not that one. I scratched my head on it for a while and decided to take the safest path I could think of, which was an expensive signal optocoupler. It's good information though (save someone some expensive magic smoke and the fine smell of aldeheydes).
For the record, the low-ball Chinese AC servo drive runs great. Aside from some cryptic operating literature, it's fine.
Thanks.