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?

Thank you,


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: https://i.sstatic.net/HwYeM.jpg

  • \$\begingroup\$ What makes you think it's RS485 interface? \$\endgroup\$
    – Matt Young
    Commented Dec 8, 2015 at 18:01
  • \$\begingroup\$ edited the question to answer, was too long for a comment. \$\endgroup\$
    – emmerich
    Commented Dec 8, 2015 at 18:10
  • \$\begingroup\$ today I'm looking into this as well. This boiler seems to me so dumb without a compatible external unit. I wanted to build my own, so that I can customize to my needs. From what I have found out at this stage is that it uses their proprietary protocol named IMG BUS. Unfortunately I could not find any other information about this protocol. \$\endgroup\$
    – Alex
    Commented Dec 14, 2021 at 9:58

1 Answer 1


Based on the information you have provided the 2-pin port is unlikely to support RS485.

  1. 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.
  2. 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 – 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.

  • \$\begingroup\$ Thank you! Indeed, the remote has no batteries, just checked the on-line manual for two models, neither have. Things become too complicated for me to reverse-engineer the whole thing. I would've liked to connect it to a RPi somehow and control it via Internet. \$\endgroup\$
    – emmerich
    Commented Dec 8, 2015 at 22:12

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