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“I am going to connect the 858 to the terrova US2.”
Why not just install the DI transducer for this 858c HD DI unit to the Terrova trolling motor and forget about using the US2?
“I still want to mount the DI transducer on the transom next to the SI transducer. Do I want to run it directly to the 858?”
Yes, if you want the 858c HD DI unit to read and display the sonar data from that DI transducer correctly than you should connect the DI transducer directly to the 858c HD DI unit.
Mounting the DI transducer right next to the HDSi transducer will cause a blockage of some of the Si sonar to whichever side the DI transducer is mounted to. If you still opt to mount the DI transducer on the transom, mount it as far to the other side of the transom as you can to minimize any blocking of the Si sonar. Also, be advised that if both units are run at the same time that there will the chance of sonar-to-sonar interference if both units are running at the same frequency.
“Could it be read through the Ethernet if connected to the 998?”
Yes, when both units receive the 6.900/6.940 software update, but until then the answer is “No”. When this does happen it will not work correctly as the 998c HD Si unit will treat the DI transducer as if it were an Si transducer.
“If I need to run it to the 858 I will need an extension to the cable, which model?”
That all depends on how much cable length you need. The DI transducer comes with a 20-foot cable, We sell 10 and 30 foot transducer cable extensions under the EC-W10 and EC-W30 part names.
“I will also need a y-adapter for the transducer and us2 connection, which one supports a DI transducer or do all of them?”
May I ask why and what you are thinking you are doing with this?
If you were to do this, the 2D sonar would come from the trolling motor while the Di sonar would be coming from the transom which I am guessing are at opposite ends of the boat. The two types of sonar (2D and DI) will not always match up. If your 858c HD DI unit is at the bow wouldn’t you want to know what is going on under the bow?