Can two Arduino Unos talk over a 2-wire M-Bus style bus (9V/7V + current reply) across 50 Meters

Hi everyone,

I'm trying to get two Arduino Uno boards talking over just two wires, a bit like M-Bus but at a lower voltage. I don't want any extra ICs, only the two Unos plus basic parts like resistors, capacitors, diodes and transistors.

The idea:

• Master to slave: the master switches the bus between 9V and 7V to send data.

• Slave to master: the slave replies by pulling extra current (about 45 mA idle, 50-75 mA for a "0"), and the master reads it with a small sense resistor.

• The slave gets its power from the bus.

A few questions:

1. Are there simple discrete circuits for this, for both the master and the slave side?

2. Is a 2V swing enough to read reliably with the Uno's ADC, or should I go bigger?

3. Is a series sense resistor the best way to read the slave's current pulses?

4. Could this work over 50 m of cable between the boards? Any tips on cable, baud rate or protect

Thanks a lot for any advice or links!