My buddy just picked up a new 788C, but his boat is laid up for winter. I told him to bring it along on our next fishing trip and we can plug it into my 778C mount to try it out. The 788C has an external GPS antenna, which he won't be bringing along. I have my 778C linked to my Garmin 498C with the Humminbird cable. Would there be any harm to the 788C by getting input from the Garmin, rather than the H/B external antenna? Or should I shut off the NEMA output on the Garmin before powering up the 788C?