RS485 and De Dietrich Diematic module

[claudiu]

I bought an TV8469 RS485 - RS 232 convertor. I'm trying to connect to the BUS between my De Dietrich central heating system and the Diematic thermostat to sniff the communication. Until now I did not succeed. From what I found on the net the communication should be MODBUS over RS485.
Any ideea where shall I look forward? Bellow are the facts/what I tried.
1. For connecting I just used 2 wired from a network cable which I put in parallel from the thermostat.
2. The thermostat is not having a ground and is not having a battery either (even if it's having a compartment for it). It is communicating with the central because is showing me the outside temperature sensor and some other information.
3. The RS485-RS232 convertor is having a + - connection (without ground). I tried to switch the polarity, in one way I receive an echo on my transmission in the other one not. The echo is a little bit strange because is not the same byte and is having an extra 0 in the receive mode (from what I read the corect mode is the one withot echo).
4. I'm using Device Monitoring Studio for monitoring and I tried different baud rates and different combinations but no results (from what I barely found on the net should be 9600 8 bits no parity)
5. If I'm looking at the voltage (with a cheap multimeter) is showing me + or - 15 V in continous Voltmeter mode or 30 V in AC mode.
I'm a little bit blocked now and any sugestions will be appreciated. Please be gentle with the remarks because I'm far from being a genius in electronics but I'm willing to continue.