Modbus RS485 - Voltage greater than 12V?
Based on the information you have provided the 2-pin port is unlikely to support RS485.


Since there is no mention of batteries in the remote control then the two wires are carrying both power and data. RS485 doesn't do than (in any configuration I've seen) and uses a differential pair, usually with a shield.
A 'normal thermostat' (bimetallic strip type) would not be detected on an RS485 network.


More than likely they have implemented their own protocol something like this.







simulate this circuit &ndash; Schematic created using CircuitLab

Power is fed out to the remote through a voltage dropping resistor. Let's say that when the remote is not transmitting data that it loads DRC+ enough to pull the voltage down to 20 V. When it transmits data it loads the line even further by connecting a resistor across the wires by a switching transistor. In our example the voltage might drop to 10 V for a '1' and back to 20 V for a '0'. The low voltage may have to be high enough to continue powering the remote - hence the 10 V.

Connecting a regular thermostat would present 31 V on DRC+ when open and 0 V when closed. Additional circuitry must be employed to detect and handle these signals.