I posed that very question to HB myself because it just didn't make sense (in my head) that the DI units couldn't at least use 2d in the US2 ...
The verbiage I quoted in post #8 is directly from the HB website ...
The below is the exact answer I received from HB ...
I'm sorry but the DI units are still NOT compatible to the US2. The model of the unit doesn't matter as it is the same from the HELIX 12 series down to the PMax series that include DI. They are not to be connected to the US2. The following link will show the units that will work and the necessary adapters needed. At the bottom of the list will have the disclaimer about the DI models.
And I wasn't even satisfied with that answer so I contacted a different HB rep and got this reply ...:
The software update will not allow the DI units to use the US2. It’s not a software reason it will not work but a mechanical. The US2 transducer will PHYSICALLY damage your DI unit if it’s connected.
Rickie