For a parking management system I am going to create an Arduino network.

An Arduino with an ultrasonic sensor will be placed above each parking space. If car is parked at specific location Arduino will inform master (PC) and also turn RED light ON.

I am not sure how I can make network with minimum wiring. I think RS485 will be suitable for this purpose.

There will be almost 200 Arduinos in a network. I have following questions in mind:

1. Should I go for RS485 protocol or something else?
2. Can RS485 support 200 devices as there will be only short communication (i.e device ID and parking status)?
3. I am planning to lay CAT6 cable. Two pairs will be used for power transmission and two pairs will be available for communication.