Two thngs to consider. Multipoint networks require that the drivers of devices onto the common transmission path be able to be tri-stated. The original RS232 specification did not provide for this capability. (Certainly today some vendors have made some drivers with this capability).

Secondly it is important that a network have a design to permit the network to be terminated in a manner to limit the signal reflections on the line. In RS232 this capability it provided by a controlled rise time at the driver and a defined receiver input impedance. The RS232 specification presents this termination design as a single point to point design. RS485 on the other hand uses a concept wherein this termination is provided by components placed at the ends of of the bus as opposed to being an integral part of the receiver this making multi-drop feasible.

A last comment.....RS485 is not a simple 2-wire network. You must provide a GND reference path. The differential pair used for the data flow path can only survive and operate with a common mode voltage if a limited amount. A GND connection is required to control this common mode voltage range.

Michael Karas