Help with tektro iox disc brakes

Hi guys, apologies for the stupidly simple question but I've looked online and can't seem to find an answer and there's nothing in the instructions which leads me to believe I'm doing something very simple very wrong. I've fitted the brake calipers and have the cables in place, the problem I'm having is when I fit the cables I have to pull them so tight that it restricts the handlebars from turning fully, to allow the bars to turn the cable has to be pretty slack which means the brake doesn't work at all. Is there something else which I should be adjusting? I've tried the pads but the cable is so slack that the pads haven't made much difference.

Thanks,

Steve