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With a taxi, just; and there are usually taxis in the station forecourt. But on foot it would be a good 30' walk, and Transmanche say anyone arriving less that 45' before the ferry departs may not be accepted.
Depending on your precise date of travel, there is also a shuttle bus from the station to the ferry terminal. That being said, the shuttle cuts it pretty fine. Timetable here: http://www.stradibus.fr/plan/?rub_code=5 (Navette Transmanche)
If I were you (and I've been to Dieppe often and am there again next week) I'd feel safer on an earlier train.
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