Harry is right: using the newer HDSi transducer of the 997 (and other newer Si units) with your 987 will only allow partial usage of the unit and the that transducer. Since the only common frequencies this unit and transducer have are the 455kHz Si sonar and 200kHz “2D” sonar frequencies; these will be the only that work with that combination of unit and transducer. If you were to purchase the original XHS-9-Si-160-T transducer that was made for the 987 unit than you would be able to use the 455kHz and 262kHz Si sonar frequencies as well as the 200kHz and 50kHz 2D sonar frequencies.
[Edit was to correct the Si frequencies available when using the XHS-9-Si-160-T transducer with the 987c Si unit.]