Very superficial wild guess: RS485 protocol, with ModBus. Very common in commercial setups to connect multiple units together.
This wizard may make life considerably easier for you if that is the case.
Alternatively, it might be the common protocol that modulates a 24V AC link. Have you checked the two wires on AC with your multimeter as well as DC? Wires and connections marked W and R are a common giveaway.
Have you examined the service manuals? They often have wiring diagrams and discuss protocols. Work backwards from your controller to see what main units it is compatible with. I would be surprised if there wasn’t a generic internal protocol that applied across a generation of Panasonic devices.
Does the device support infrared or WiFi control. Check what pins are used for that.
Examining data sheets for chips the wiring connects to at either end can often yield clues. Most chip vendors release application notes and the examples given are often used as a template for final design.