They's right. You can not run SI through a US2. You can hook it to it with the US2 H'bird adapter though and get regular 2D non DI/SI signal. If memory serves, and depending on your model, I seem to recall you could hook 2 transducers to the Hbird and use the unit itself to switch between them. I know my 797 was not capable of that, but the 997/998 might be. If not, you'll either need the splitter (think it's AS-Y or something close) or the AB switch.

Wannabe...