So assuming the HDSI will be moved to the bow with the 998 ...
And assuming the 2d xducer will still be used for high speed ...
You will need the M 9 SIDBY Cable to connect the 2d xducer and the MEGA xducer to the HELIX ...
https://www.humminbird.com/Products/9-M-SIDB-Y/
You will need the AS EC QDE Ethernet adapter cable for the HELIX ...
https://www.humminbird.com/productDe...eid=728&id=868
You will need an Ethernet cable long enough to reach between the HELIX and the 998 ...
https://www.humminbird.com/productCa...74&facets=7319,
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The HB TS3 Switch cannot be used with the MEGA unit and still have DI from the MEGA xducer (there is no wire channel in the HB TS3 Switch that corresponds to the DI wire channel in the MEGA xducer cable) ...
Rickie