Nothing. You should have all the cables you need to make it work (more on that below). There are some cables that come with the InterLink that will connect to both your 898c Si unit and to the 728 unit. You have three ways of doing this:
1.) Connect the GPS Receiver you have to the InterLink and the InterLink to each units COM connector (where the GPS Receiver plugs in). Advantage: each unit can use the GPS Receiver without the other unit having to be powered on. Disadvantages: neither unit will have the faster 4x GPS refresh rate and depending on where the GPS Receiver is located; the waypoints will be off for one or both units.
2.) Connect the GPS Receiver to one of the units using the AS-YC “Y”-cable that came with your 898c Si unit. The GPS Receiver will plug into the connector labeled “NMEA-COM”, the InterLink plugs into the connector labeled “ACCY-COM” and the third plug is connected to the COM connector on the unit. The second unit gets connected to the InterLink with the supplied cables to that unit’s COM connector. Advantages: The unit that the GPS Receiver is connected to will have the faster 4x GPS refresh rate and will have more accurate waypoints (if the GPS Receiver is located close to the transducer that it is using). Disadvantages: The primary unit (the one that the GPS Receiver is connected to through the Y-cable) will have to be powered on for the secondary unit to be able to use the GPS Receiver. Only the primary unit will have the faster 4x GPS refresh rate. The waypoints will be off for the secondary unit (Assuming that the GPS Receiver is located close to the transducer that the primary unit is using).
3.) Okay, for this you would have to purchase another GPS Receiver. Both units have GPS Receivers connected to them through an AS-YC Y-cable and have the GPS Receivers located close to the transducers that they use. Advantages: both units have the faster GPS refresh rate. Both units can use GPS independently of each other. Waypoints for both units will be more accurate. Disadvantages: cost of the second GPS Receiver and its installation.