Preparing for OpenTherm Monitoring

More and more my weblog is becoming an ‘archive’ of what I did and, more important, how I did it. I know that I can find that specific picture or other piece of information I’m looking for on my NAS or a local HDD somewhere, but a search on my own weblog is much quicker, and it works too, more and more. So I decided to write down every step I take with this OpenTherm project on my blog and in more detail than I’ve done before – I think I’ll thank myself for that later ūüėČ

Before heating up the soldering iron, I spent some time reading the OT Protocol Specification v2-2. I won’t go into all the details, but here are some important things to know.

OpenTherm is a point-to-point communication system between boilers and room units (the thermostat). The room unit is supposed to calculate a heat demand which is sent to the boiler; the boiler should act on the information provided (more or less heating) and the boiler can also send back various types of information like diagnostics and other useful stuff (to display on the thermostat LCD, for example).

The physical way in which the boiler and thermostat exchange information over the 2 wires is that the boiler transmits a signal by changing the current (high=17 .. 23 mA, low=5 .. 9mA), while a thermostat does this by changing the voltage (high= 15 .. 18V, low = 7V).  The boiler is also supposed to provide the room unit with power, so no batteries should be needed.

The encoding method that’s being used is called Manchester, where a transition from low to high represents a ‘0’ bit value and a transition from high to low is a ‘1’; more about Manchester code here.¬†OpenTherm uses a bitrate of 1000 bits/second.

An OpenTherm frame consists of 32 bits e.g. 4 bytes and those 32 bits are transmitted with a leading start- and a trailing stop-bit. These 32 bits contain information like parity, message type (read, write, read-acknowledge, invalid data), data identifier and 1 or 2 (used) data bytes. In a OpenTherm conversation it’s always the thermostat (room unit) that acts as master; hence, the boiler is the slave. Communication is therefore always initiated by the thermostat, not by the boiler and the latter is supposed to reply within 800 ms. Furthermore, the protocol documentation states that a master must communicate at least every second. (question for myself: how does this relate to a wireless connection between thermostat and boiler?)

That’s enough info for now; lets have a look at the first thing I want to do: monitoring OpenTherm traffic with the Elektor OpenTherm Monitor. Lets have a look at the¬†schematic of the Elektor Opentherm Monitor (OT Monitor) and what has to be done to connect this OT Monitor to an Arduino (or a JeeNode, in my case) – cause that’s the goal of this first step.


(the image can be a bit blurry due to the resizing; clicking it will reveal a better image)

The right side is where the builder of the OT Monitor is supposed to hook up his/her RS232 port. But that’s not going to happen – at least not if I can manage to modify the Elektor OT Monitor to this schematic, which¬†I found on a site I mentioned before:¬†

I can clearly see the modifications on the right side of the schematic (duh), but to make sure I don’t forget or oversee any of the modifications, I opened the original Elektor schematic in MSPaint and started modifying the original to get to the end result that I need. All modifications were done in red, so that it would become much clearer to me what had to be changed on the original PCB and what additional components had to be added (on a separate piece of perfboard):


It might look ugly or silly (or both), but it works for me and that’s all that counts, right? It reduces the labyrinth..

So, 1 diode needs to be replaced by a wire and the other connection with a diode will (must) not be used anymore – that takes care of the PCB-side of what has to be done. That should not be too hard.

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