How has this Daikin control signal been encoded?
As Marko answered, it is the Daikin proprietary P1P2 protocol. At the lowest level it is a 9600 Baud serial-like interface based on the Japanese Home Bus System (HBS, ET-2101). Technical details of this standard can be found in chapter 4 of the Echonet specification. HBS uses a variation of bipolar encoding (alternate mark inversion, https://en.wikipedia.org/wiki/Bipolar_encoding), but the pulses only take half of the bit time. So every falling edge in the signal represents a 0, and every "missing" falling edge represents a 1. Every byte is coded as a start bit (0), 8 data bits (LSB first), 1 parity bit (even), and 1 stop bit (1). Oscilloscope pictures which show the signal are made by another user and published on http://www.grix.it/forum/forum_thread.php?ftpage=1&amp;id_forum=1&amp;id_thread=519140&amp;tbackto=/forum/forum_discussioni.php?id_forum=1. As it is a two-wire interface, devices should not write on the bus if any other device is writing. The HBS standard has an advanced collision detection and priority mechanism. Daikin does not follow the HBS packet format or HBS timing specification. In a 2-device set up of a Daikin heat pump and a thermostat, the devices seem to simply alternate turns in writing to the bus.

A power feed for the thermostat is also added to this signal: the average potential difference between the two wires is 15V DC.

More documentation and how to read and decode the data yourself is available on github: P1P2Serial.