A missing piece of information... I did not say that the final intention, if I succeed with the decoding, is to get in the way and read and feed signals at will from a Rpi.
The wires carry for sure: power (24VAC+GND), 1-remote wire, and 2-rain sensor. Then I assume the five remaining may be one for Vcc and 4 for control of 15 zones plus pump. I am guessing the remote signal is a pulse train riding on one of the 24VAC lines. The control side may be based on shift registers...