DM5 Ext Trigger #1 (Hi Hat) dead?

I just bought a used DM5 brain to replace my original, which had lost input on 5 of its external triggers.

The new unit appears fine APART FROM input #1 (the hi-hat), which I can't get a response from. I've tried using a known "good" pad (i.e. one which works fine as a Tom trigger), but I do not see Trigger #1 highlighted on the display when I hit the pad.

Does this necessarily mean that the trigger input is faulty, or is there some setup specific to the hi-hat trigger that I may have missed? How can I check definitively whether trigger #1 is working?
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