According to the Minn Kota web site you would need the MKR-US2-8 Humminbird 7 pin Adapter Cable.
http://store.minnkotamotors.com/dyn_prodlist.php?k=418661Now, when you use the US2 sonar you will not have any Si or Di sonar as the US2 transducer is only a 2D sonar transducer. You will need to set the Transducer Select menu to the Dual beam menu setting.
Your only choice to connect the 797c2i Si unit to any other unit is via the InterLink.
My $0.02 (not Humminbird’s, though it could be why they did not give you an immediate answer):
Due to changes in the software you will have to find a 900 or 1100 series unit that was built with some old version of software for the best chance of the two units working with the InterLink. The InterLink has the best chance of working of both units are running the same level of software (the beginning number needs to be the same). The last software update for the 797c2i Si unit was 5.450. Currently we are producing 900 and 1100 series units with the 6.610 version software. You would have to find a unit that was produced 2011 or very early 2012 that has a 5 level software. There is no way to know this without powering the unit up and looking at the software version or by looking at the serial number on the unit (we put serial numbers on the outside of some boxes now). That is not to say that a brand spanking new 1198c Si unit with 6.610 software will not work with the 797c2i Si unit and InterLink but what I have been seeing is that the chance for this to work is not good. Buying a new unit and loading a software version that is older than what the unit was originally produced with could cause it to not work at all. It would have to be returned to Humminbird to be reprogrammed and I have heard that this would not be covered under the warranty.
If you buy the InterLink and new unit you will have all of the cables and connections that you need.
You could run both units off of the 797c2i Si’s internal GPS Receiver but it would always have to be on to do so. If it was turned off or the InterLink connection failed you would be without GPS at the console. It is preferred that both units have their own GPS Receivers. This gives you a backup in case one fails. Both units will still have GPS if the InterLink fails and you will have more accurate waypoint marking (and will be able to more get back to waypoints more accurately) with both units.