Myles Eftos[Eddi] made a great find! This is a discussion on EEVBlog on decoding the interface on an LG system. While I think the comms protocol is different, there is a suggested schematic for a Dual Comparator Serial Bus transceiver that looks like it is exactly what we need. I have ordered some LM393s - I'll build the circuit one they arrive. https://www.eevblog.com/forum/projects/where-to-start-trying-to-decode-a-data-stream/25/
In the mean time, I'll pull the second unit I have out and see if I can get it running in slave mode. That was I can listen to it's TX pin, and see if I can work out what it says.
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I did a very similar project on my Chinese minisplits back in 2009.. in my case i wanted to run these from my Home automation system and be able to turn thyem on and off as well as read their current parameters.. it turned out that the single wire bi-directional protocol from the wall control to the indoor units is the SAME as the wireless remote.. so its a pulse-distance-encoded protcocol.. in my case its 0.768 milliseconds for a 1 and I think its 0.36 for a 0, it sent a string of 8 bytes first forward then backwards as opposed to using a CRC / checksum. it didnt follow a normal baud or serial form like you would expect.. I used a zbasic.net chip and built a board to talk to them and translate it to ethernet for my Homeseer home automation system.. its been a long time but I can dig up the details if you are interested.. the trick was that it follows the same conventions as a IR wireless remote. Fujitsu does make a Modbus interface device for their units and it works slick as can be.. no brainer to use that on my new build systems im working on for local control.. i dont think fujitsu has a public API to control their units so the Modbus device and a raspberry pi could be your ticket.
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This looks exactly like serial protocol that I reverse engineered for Fuji Electric AC unit. I might have some answers for you. https://www.manualslib.com/manual/1239980/Fujitsu-Aoyg30lat4.html?page=36#manual is where I started. The diagram should reveal how 1-wire communication is converted into Tx and Rx of the MCU in the indoor unit.
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