SR8 and M+ redundancy doesn't seem to work

[Scottand1000]

I have a SR8 Pro, and I am trying to set up a M+ as a redundant Rx. I can't get the SR8 Pro to *see* S.Bus input from the M+. What doesn't work is M+ connected to SR8 S.Bus IN.

For completeness, SR8 Pro and M+ using firmware 2.1.12. Tx using OTX 2.3.15


I have a spare R8 Pro (not sure of its firmware rev). I tried the following combinations.


M+ connected to R8 Pro S.Bus IN -- works (w each Rx bound to a different model i.e one Rx green light, the other Rx red light). This verifies M+ and its soldered cable is working.
SR8 Pro S.Bus OUT connected to R8 Pro S.Bus IN -- works (w each Rx bound to a different model i.e one Rx green light, the other Rx red light). Interestingly, the SR8 Pro S.Bus output seems to work OK.

--but--

R8 Pro S.Bus OUT connected to SR8 Pro S.Bus IN does not work.

So, both the M+ and the SR8 Pro can talk to the S.bus IN port on the R8 Pro, but neither the M+ or the R8 Pro can talk to the S.Bus IN port on the SR8 Pro.

I have a second SR8 Pro (firmware rev 2-1-10). I repeated all the above with the second SR8 Pro. Same results as above.

Any idea why the SR8 Pro doesn't seem to take S.Bus input?

Thanks, Scott