Below some notes regarding the RTD-LT/CA and controls with Modbus (please read the article How to install the RTD-LT/CA first).
Q: I'm writing to Holding Register H0003 (Operation mode = {Auto, Heating, Cooling}), but the indoor unit doesnt react.
A: Please, first switch the unit On using Holding Register H0004.
Q: Sometimes Register H0004 – Modbus (On/Off space Heating/Cooling) doesn't turn off my Altherma. Only after reconnecting power supply it is set to OFF.
A: The Modbus On/Off signal operates on a 'Last Touched' function. This is because the On/Off state of the Altherma Unit can be adjusted from Modbus, from Remcon and from Input commands on Input S1 and S2.
If H0004 = 0, then to write an OFF with Modbus the register must see a change of value. Therefore it is necessary to write a 1 and then write a 0. This situation only occurs in the case of mixed control of the Altherma system using Hardwired or Local On/off commands with Modbus commands.
Q: How can I set the Holding Register Update Mode?
A: The corresponding Update Register is the Control Register + 200. In this corresponding Update Register you can set any value between 1 and 4 to select the update mode.
Example:
Register to select Operation Mode: H0003.
Corresponding Update Register:: H0203.
Setting the value in H00203 to "1" means that the update mode fo the Operation Mode is "Central".
Q: Is there any register that shows the actual ON/OFF state of the unit?
Q: If relay 1 is set to signal RUN then you can read the relay 1 (R1) register value H13101 to check whether the unit is running (H13101 = 1) or Off (H13102 = 0). H13102 reflects the R2 value.