Hi Harald,
I made some progress with the Nokia UltraSite and managed to fix its backplane, so at least I was able to continue with reintroducing hopping control with the first 2 TRX slots of the cabinet.
During this process I realized that apparently even multi-TRX support is broken, which was working on the old OpenBSC codebase in 2016. If only the first TRX is configured the BTS bootstraps fine, but when a second TRX is configured, the unit goes into this weird state: the first TRX loops through configurng and waiting for LAPD states and stays in configuring state, while the second TRX is stuck ad supervisory state where TRX test is possible (it does pass the test).
I used my old multi-TRX config file that actually worked with UltraSite (and MetroSite) where the second TRX only has TCH/F so nothing fancy, no hopping or anything like that.
I wonder if my modification of moving the RSL bootsrap to after CONF_COMPLETE somehow changed something, maybe the BTS requires the RSL LAPD links to be up during the CONF state? Or the new FSM lchan refactoring might have introduced this in 2018...
So apparently we have some more basic issues here then hopping :-)
Regards, Csaba