plisken:
pdf datasheet here: http://www.titanproducts.com/download/datasheets/62.pdf
You call that a datasheet? More like a sales brochure. It says nothing about what "volt free" inputs are.
Is anybody forcing you to use it? It seems like a complicated way to read 4 digital inputs.
plisken:
Although the sheet says “volt free”, I am measuring them at 5v.
Maybe they have pullups on them. Put a multimeter (showing voltage) on the "DC Output" pin and try shorting the pairs of pins together.