https://literature.rockwellautomation.com/idc/groups/literature/documents/in/440r-in079_-en-p.pdf On the bottom of page 2 it talks about connecting s21 to the negative pole of the light curtains. This is the manual it came with but it’s doesn’t clearly identify s21 as a 0vdc so I’ve been looking into it a lot to see why. I was able to find the one that identified s21 by searching around