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- thermostat: NCU-2te
- heater: Nobo Fjord NFK4N-20 2000W
I disassambled the unit and identified the ground, power lines, and it was easy to spot the SCL, SDA text overlays.
The NCU and heater are communicating via I2C. I recorded the communication with Saleae logic analyzer. The NCU is always the master. There is only one master on the bus. There are two slave devices on the bus:
- The realtime clock placed on the NCU PCB
- NXP PCF8523 realtime clock and calendar.
- address 0x68
- The NCU initializes the realtime clock, sets the date and time, weekday, etc.
- it is irrelevant to me.
- The heater itself
- address 0x01
- I identified 3 different commands:
- 0x01: set target temperature
- 0x03: read environment temperature
- 0x05: get heater state from the heater
With this command, we can get the status of the heater. The heater is either ON or OFF. I observed thet the heater is using PWM to adjust heating power. The period of the PWM is 100 seconds.
| NCU | Heater | |
|---|---|---|
| start condition | ||
| Setup write to 0x01 | ACK | |
| CMD 0x05 | ACK | |
| repeated start condition | ||
| Setup read from 0x01 | ACK | |
| NACK | STATE | 0x00 when the heater is OFF/not heating, and 0x01 when the heater is ON and heating |
| stop condition |
The heater has a temperature sensor inside, at the very bottom, farthest from the heater element.
| NCU | Heater | |
|---|---|---|
| start condition | ||
| Setup write to 0x01 | ACK | |
| CMD 0x03 | ACK | |
| repeated start condition | ||
| Setup read from 0x01 | ACK | |
| ACK | B0 | temperature in milidegrees °C, LSB |
| ACK | B1 | temperature in milidegrees °C, MSB |
| ACK | B2 | seems to be always zero |
| ACK | B3 | seems to be always zero |
| ACK | B4 | ??? different every time, could be CRC |
| NACK | B5 | ??? but I couldn't find what polynom |
| stop condition |
The heater is doing all the work to reach, and keep the target temperature. The NCU's job is to send the desired temperature. The heater is receiving the target temperature, valid from 7°C to 30°C. Also there is an OFF option, where the heater is letting the temperature to drop below 7°C.
| NCU | Heater | |
|---|---|---|
| start condition | ||
| Setup write to 0x01 | ACK | |
| CMD 0x01 | ACK | |
| B0 | ACK | 0x08 when the device is set to a temperature, 0x0B when it is set to OFF |
| B1 | ACK | target temperature in milidegrees °C, LSB |
| B2 | ACK | target temperature in milidegrees °C, MSB |
| B3 | ACK | seems to be always zero |
| B4 | ACK | increases by 84 with 1°C target increase |
| B5 | ACK | decreases by 64 with 1°C target increase |
| repeated start condition | ||
| Setup read from 0x01 | ACK | |
| NACK | STATUS | 0x17 when setting is valid |
| stop condition |