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,
P.S.:
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
Best Answer
Based on the information you have provided the 2-pin port is unlikely to support RS485.
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.