Thanks for thinking along.
In the meantime all obstacles are solved
except for one detail.
As I thought it was not because of the wiring
and what is related to it because the information about the depth from the DST810 could be seen on plotter and app and is sent via the same sensor and cable to the backbone and from the speed on the ST60 displays.
Problem 1: B&G Zeus˛ plotter shows depth but not speed. Via settings/network/ I saw that the Airmar DST810 is not set as default for speed but it is for depth. Strange because no other depth or speed sensor was ever connected before.
Problem 2: the Raymarine Seatalk 60 displays only show speed but not depth. This was changed by switching the depth display from master to repeater mode.
The only problem is that the depth is displayed in ft. and not meters. Setting this can only be done in master mode but it is not included when switching back to repeater mode. Is this too much to ask?