I have 3 GlobalLink 520s and I'm trying to use the digital inputs to monitor the state of a relay. On 1 unit, both digital inputs are normally high and I'm able to use a relay to pull them low by attaching them to ground. On the other 2 units, digital input 1 is normally low and digital input 2 is normally high. According to the manual, both digital inputs should be normally high. What would be causing the digital input to be normally low on those 2 units? There is nothing connected to the digital input that is normally low.