u/Dull-Shallot3646

Can someone explain to me the operation of a DI input or let me know if my thinking is correct. When the Digital input contact is open I am seeing 4.9 vdc from sig to 0v and 4.9vdc from sig to ground. When the contact is closed the voltage goes to 0v across the two terminals as does any closed contact or switch… but what I don’t understand is when the contact is closed I get 0vdc to ground on each terminal. Do the contacts create a short when closed and that why the voltage drops to 0?