I wonder if an92626 ever found a solution to this problem?
I am facing a similar issue with the machine I'm currently working on. 7i92+7i77 running on 2.7.14 - Debian Stretch. I changed everything around in trying to find the defective wiring/encoder but in cannot get enc0 and enc1 on the 7i77 to work. b input remains dead on those two. I have even changed the a and b inputs on the terminal at the 7i77, and still b phase show nothing. wired a different encoder (new, that I checked on the oscope as well), still nothing on b.
If I plug the same encoders into enc2 - 5, it works as it should.
going to change the 7i77 later today as a sanity check.
Henk