I have a boiler (Immergas Eolo Star) which has 2 pins where you can connect either a normal thermostat or a "remote control unit" which can also give more detailed information (self-diagnosis) about the boiler. This made me think that it has implemented some communication protocol over RS485, maybe Modbus and I could connect to it via a RS485 adapter and have fun. Today I got the time to have fun with it but to my surprise the voltage on those pins is (measured) 31.8V DC / 7mA. This confuses me a little since as far as I know, RS485 operates at max 12V.
Do you have any ideas, should I add something between the adapter and the pins so the voltage level should be lower? If so, what should I put there?
Why do I think is working on RS485?
The fact that the "remote control unit" can be connected via the same 2 pin interface as a normal thermostat; also, I've checked that make of boilers and for other boiler family the "remote unit" had clearly specified that it's working on RS485. For mine this is not clearly written somewhere unfortunately.
From the manual I get only this:
"Digital Remote Control Device with climate chronothermostat function (Fig. 1.6). In addition to the functions described in the previous point, the Digital Remote Control panel enables the user to control all the important information regarding operation of the appliance and the heating system with the opportunity of easily intervening on the previously set parameters without having to go to the place where the appliance is installed. The Digital Remote Control panel is provided with self-diagnosis to display any boiler functioning anomalies. The climate chronothermostat incorporated into the remote panel enables the system delivery temperature to be adjusted to the actual needs of the room being heated, in order to obtain the desired room temperature with extreme precision and therefore with evident saving in running costs. The chronothermostat is fed directly by the boiler by means of the same 2 wires used for the transmission of data between boiler and chronothermostat."
As per installing the remote control unit: "Digital Remote Control or On/Off chronothermostat electrical connections (Optional). The operations described below must be performed after having removed the voltage from the appliance. Any thermostat or On/Off environment chronothermostat must be connected to clamps 40 and 41 eliminating jumper X40 (Fig. 3-2). Make sure that the On/Off thermostat contact is of the “clean” type, i.e. independent of the mains supply; otherwise the electronic adjustment card would be damaged. The Digital Remote Control must be connected to clamps 40 e 41 eliminating jumper X40 on the P.C.B. (in the boiler), (Fig. 3-2)."
Here's the wiring diagram: http://imgur.com/GC8i9Nx