I have a 598ci si hd, with XNT-9-SI-180-T model transducer.

If I were to add a 2nd used unit at the bow, sharing the transducer, using the US3 switch, which HB units are compatible with this transducer?

Thanks

Every Humminbird core unit Side Imaging model and every Humminbird Dual Beam 83/200 kHz 2D unit.
The core units have number model names plus the Helix 5's.

Since that transducer is shorter than the HDSI model, it will not produce as sharp of an Image, but it will work

That switch does NOT share the transducer, you select which one unit to connect to the transducer.

So, could I use it with any mid-level unit with DI or basic chart/sounder?

And, so with the US3 switch, I could switch between two units and use the same transducer, just not two at the same time, correct?

Also, thanks for the help.

No it will NOT work with a DI unit for Down Imaging. ONLY SI or 2D units.

Yes the US3 is a UNIT SWITCH, you select the unit to connect to the transducer. It is a powered contact, so it needs 12V to operate.