Experimenting with Modbus protocol


I'd like to experiment with Modbus protocol and libraries but have to wait for a few weeks before I get RS485 modules.

Meanwhile, I could play with some Arduinos laying around : I plan to create a bus (Tx-Rx) with the slaves, connect it to the master (Rx-Tx, crossed), and test libraries over SoftwareSerial. I have been searching for hours on the web, but did'nt find any examples of such a test setup for Modbus communication (direct connection, no RS485 adapter). I'm pretty sure it would work, but I'd like a confirmation (or not...). As I understand Modbus, slaves are 100% passive so there can not be any collisions between frames.

At this time I just want to experiment with protocol between Arduinos (not Arduino and PC over USB for example).