Updated P4 to latest firmware and try to get Line monitor port to work.

This is what I have done:

Connected dc 12v plug to Line monitor port, checked that transformer is working and power is on.

Turn on power cut monitoring on from GCC system page (use linemon input)

When I save changes it goes instantly to "power cut" state.

Try this with two tranformers but result was same. What I should try next?