For connection between CTC GSi and Homeassistant (testet with GSi 8 and GSi 16)It probably works on EcoHeat 400 series as well. I haven't found all the modbus registers yet from the lists in the pdf files
RJ12 cable cut in half an connected to NodeMCU ESP32
Pinout for the RJ12 cable connected to the display of the CTC heat pump
1: GND
2: 5V
3: Not used
4: RX GPIO 16
5: Flow control pin GPIO 5
6: TX GPIO 17
Using Esphome it is possible to read and write modbus.