I/O Board accepts "5V TTL, open/closed circuit" - So it will accept the closing of a circuit with no voltage?
There's a somewhat more informative section in the Äkta Avant User Manual.  Your E9 module is an accessory for the Avant.
From page 147:

If you leave a digital input unconnected, it will read as a digital high (logical level 1.)  You have to intentionally pull it to ground to make it read a digital low (logical level 0.)