This README is based on Google translations of documentation in a post by krzysztof1111111111 on https://www.elektroda.pl/rtvforum/topic3499254.html and the ZPS documentation in this repository. A big difference with the ZPS situation is that in a PCWU setup the heat pump and controller are already connected using RS485. Thus there are two modes of possible communication:
Default communication parameters are 38400 8N1. A connection can be made by cutting wires between the controller and the heat pump and connecting a RS485 UART in parallel to the A and B lines. This was tested using an Elfin EW11 RS485 to Wi-Fi device which can be acquired for very little:
This device supports a wide range of voltages so the 12V + and - lines were also connected in parallel. If your RS485 device needs a different power source, simply don't cut the + and - lines or connect them back together to supply 12V to the G-426 controller.
The documentation for the protocol between the PCWU and the G-426 controller has been largely documented by krzysztof1111111111 in Geco Proto.docx. This document has been Google translated from Polish to English.
This mode of communication is best suited to inspect and document the protocol spoken between the PCWU and G-426. It can also be used to 'overhear' relevant stats and use those in a 'read-only' fashion. For normal operation though it is probably better to use direct communication with the PCWU as described below. In theory it should also be possible to disconnect the G-426 and take over the communication with the PCWU entirely, but this has not been tested yet.
The mainboard of the PCWU has two RS485 connectors. By default the G-426 controller is connected to the 2nd port while the 1st port is free. In my case Hewalex even provided an empty screwable connector in that spot and I could just unplug the connector from the mainboard and use a small part of wire from the G-426 connection cable. Just make sure to follow the colorcoding as seen on the 2nd RS485 port. Then connect your RS485 to Wi-Fi device to the other end of the piece of wire making sure that the A, B and 12V lines are connected in the right order.
The big difference is that this way of connecting allows the PCWU to communicate with the G-426 controller in its usual fashion while at the same time it is possible to read and write to the PCWU using the RS485 to Wi-Fi device. The G-426 controller settings menu allows to set the baud rate, physical address and logical address for the dedicated RS485 port. Recommended values are 38400, 2 and 2. The protocol is the same as describe above.
See https://github.com/mvdklip/Domoticz-Hewalex/blob/master/hewalex_geco/devices/pcwu.py for an overview of status and config registers.