Reverse engineering a Fujitsu Air Conditioner Unit

I want to automate my air conditioning. Unfortunately, the Fujitsu remote controller we have doesn't have IR. It does have serial though...

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We have central, reverse cycle air conditioning that is controlled my a wired remote control unit. When I mentioned I was thinking about how to control it via my phone, my wife actually thought it was a good idea, so I made a start before she changed her mind. Building on my Garage Door opening project, I would like to use an ESP8266, talking MQTT to Everything will be over TLSv1.2, and I'll continue work on the web configuration interface.

Since this remote control unit (a UTB-YUB/GUB/TUB) doesn't have infrared, I couldn't just point an IR blaster at it. After some googling, I found a number of installation manuals and discovered the remote uses some sort of 1-wire (as in an actual single wire, not the protocol invented by Dallas Semiconductors) to do communication.

A few ebay searches later, and I found new remote control unit for $60 (from a supplier 20 km away), so I have a test unit that I can play with.

First step: Work out how this thing comminucates with the outside unit...


Excel spreadsheet showing different bitstreams depending on the settings of the outdoor unit

sheet - 35.45 kB - 02/25/2017 at 04:48


  • The protocol from the outdoor unit

    Myles Eftos02/25/2017 at 05:28 2 comments

    So, I think I've worked out the meaning of the bitstream coming from the outdoor unit!

    On my day off, I took the unit of the wall, got me some coffee and setup shop in the hallway, oscilloscope in hand.

    I must admit, I'm still getting used to using the oscilloscope and I'm sure there is a far better way to do what I'm trying to do, but I found that if I probe the RX pin on the CPU, with the 'scope set to single trigger mode and keep hitting the start button, I'd eventually align the waveform at the start of the cycle. After that I used the onscreen rulers to work out the gaps between the pulses. I then wrote them down in to this spreadsheet. I'd change a setting, take a new set of readings, and repeat until I had covered enough states that I could get a complete picture of what was going on.

    Looking at the data, I could start to see some patterns.

    1. The shortest spacing was around 2ms (some longer; some shorter)
    2. The RX pin is idle low, and there is always a high transition to represent the start bit
    3. There seems to be a low transition to represent a stop bit
    4. There is 9 bits between the start and stop bit (except for the last set)

    It's starting to look like a straight up serial transmission, except the idle state, start and stop bits are inverted, so unfortunately the built in serial protocol decoder wouldn't read it.

    Next I need to find the bits that change between each state.

    The power bit was pretty obvious: there was only one bit that was different when the power was off - the 68th bit.

    Looking at the rest of that byte, there was a pattern developing in the next 3 bits - they seemed to change when the settings changed. Taking LSB first, Fan only mode is represented by 0x01, Humidity mode (Yeah - I don't know what that is either) is 0x02, Cool mode is 0x03, Heat mode is 0x04 and Auto mode is 0x05. The next three bits represent the fan speed: Auto 0x00, Speed 2: 0x02, Speed 3: 0x03, Speed 4: 0x04. But was was the ninth bit?

    Having a think about serial, it's could a parity bit. By summing the number of bits, it became pretty obvious it was odd parity. I checked this against the other bytes, and it checked out - now we are getting somewhere!

    Looking at the next byte, it was clear it was changing with the temperature. I purposely looked at the lowest possible setting for the temperature (15deg) and the highest (30deg) and it was here I was lead down the garden path a little. Reading up on other people's efforts at reverse engineering air conditioner units, this is a fairly common range. Many of the IR transmitters represent this as a 4 bit number, where 0x0 is 15 and 0xF 30. Unfortunately, I couldn't for the life of me work out how that mapped to the numbers I was seeing.

    It turns out, this system uses a 5 bit number - feasibly being able to represent 0 - 31 degrees. Bits 6 and 7 are always 0, and bit 8 is the "economy" settings.

    There is four unknown bytes, and one block that seems to be make up of 5 bytes. My guess is one of the unknown bytes is reserved for errors, and one is a serial number of some sort. I have no clue what the other two could be for, and I'm quite confused by the last, short byte.

    But this is definitely progress!

    I did a final check the get some timing on what is transmitted, and there seems to be three windows of roughly 212ms each. The first from the outdoor unit, the second transmitted by the remote control, and I'm guessing the third is for a slave unit.

    To build a test harness, I'll need to bit-bang the data for 212ms, then set the line to high impedance for 424ms. This will hopefully allow me to get the remote control to work on my bench. Once I can get the remote to work, I can analyse what it is doing. Next, I'll simulate that as well, then set the remote to slave mode and work out that part of the protocol. Once I have the three parts of the protocol nutted out, I can just simulate the outdoor unit, connect the spare remote controller as master, and the microcontroller will become the slave. Easy!

  • Baseline communication

    Myles Eftos01/16/2017 at 22:03 0 comments

    I took the remote unit off the wall again, and this time removed the signal wire fro the remote and attached it to my Oscilloscope.

    And this is signal that comes from the outdoor unit.

    I'm not sure if I stuffed up my reading the last time, but it looks like the pulse width is 2ms.

    Really, I needed to replay this and see if I could get my test unit to initialise. I thought about using an Ardiuno, so I googled bit banging serial to see the best way to do it. One of the results that caught my eye was another Hackaday article entitled "Introduction to FTDI bitbang mode". I had literally just cleaned up my workbench and found a FTDI module. Perfect!

    I knocked up a little circuit that drove a transistor from 0V to 12V, and adapted the code from the article to control the FTDI modules CTS line. I had to reduce the sleep time to 1.8ms to adjust for kernel context switches (I'm guessing) while talking to the adapter. I got it pretty close to 2ms though.

    I wired it up to the controller, and got one step closer - now instead of timing out and flashing C0 12, it just sits flashing "9C" forever.

    My guess? This communication protocol works on one-wire - I'm not releasing the line, so the remote never gets a chance to send a response. It looks like I'll need some sort of tri-state buffer, so I can set the line to high-impedance after I've sent the preamble.

    I was curious to see if I could get any other clues to the protocol, so I started poking around the big chip on the PCB. One of the pins receives the same signal the signal line does, except it's inveted and 0-5V! I went and looked up the chip (it's a UPD78F0393 from NEC - I'm so glad the remote manufacture labelled all their chips nicely), and that pin (#75) is labelled RXD0. That sounds like a serial receive line to me!

    Pin 76 is labelled TXD0, which I'm guessing is the transmit line. This should make decoding stuff way easier, because I'll be able to see what is actually being transmitted and received separately. Win!

    I'm going to try and trace out the front-end to this - so far I see a NJM2904 (an op-amp) is on the path - my guess is that is the thing inverting the signal and driving it to 12V. Tracing this circuit out should allow me to build a compatible circuit from my microcontroller.

  • A test unit arrives

    Myles Eftos01/16/2017 at 21:32 0 comments

    So my test unit arrived!

    I get it on my bench, and test out my theory - if I'm right, it should boot up and start sending commands when the buttons are pressed.

    I was wrong.

    The unit just sits there flashing "9C" for a couple of minutes, then failing over to a "C0 12" error. The Oscilloscope was no use either - I just saw a constant 12V on the signal wire.


    Looks like I'll have to pull off the real wall unit.

    Using some wire and alligator clips, I extended the wires so I could reach them with the scope.

    This time I got somewhere - I could see a signal!

    The pulse width is around 1.04ms, going from 12V to 0V. Weird.

    I go distracted for a while trying to decode the protocol - is there start bits and stop bits? What about a parity bit?

    I knocked together a quick D3 script (I'm a web developer, remember - I use web technology for a lot of this stuff because that is what I'm used to) to display the wave form. First, I wrote a ruby script that created a CSV file of just the transitions. There are two entries for each transition - a 0V and 12V value - so the graph ends up looking like a binary stream.

    I then wrote another script that aligned the stream so each pulse was exactly 1.04, and each pulse hight was 12V or 0v. Finally, I scaled everything so the pulse width was 1, as this made reading the graphs easier.

    I ended up with some pretty graphs like this:

    There was still a problem though - I didn't have a baseline for the communications.

    I knew that the control unit didn't send any data unless it was connected to the outside unit. I also knew that changing the temperature changed some of the bits in the data stream, so clearly there was some half-duplex serial communication going on. I needed to find out what the outdoor unit sent to initialise the control unit...

  • How does this thing work?

    Myles Eftos01/16/2017 at 21:08 0 comments

    I've never done any reverse engineering before, but spurred on by this recent Hackaday article, and this article I found I thought I'd give is a crack.

    The first issue: I had no idea what the model number was - it's not written on the unit, nor on the instruction manual. So I just googled for Fujitsu airconditioner remote, hit image results and looked for one that looked the same. Once I found it and clicked through to the source page, I found out that it is a UTB-YUB/GUB/TUB (There are three model numbers depending on where in the world you are).

    I found a supplier on ebay (who was actually Melbourne based), who had a new remote unit for $60, which I bought as I wanted a test unit on my workbench - mainly because trying to test things using the unit on the wall would be really annoying.

    While I waited for it to arrive, I continued googling to find as much info as I could about it. Thankfully, a number of airconditioning repair places have their installation manuals online. Reading though the them, it was clear there was a three wires that connect the remote to the outdoor unit - +12V, GND and a signal.


    Now, I need to work out what this signal wire does.

    My first hypothesis was the remote unit worked a lot like an IR remote - every button press sent the complete state to the outdoor unit. If this was the case, it should just be a matter of hooking up a DSO (I have the LabNation SmartScope), copying the signal and replaying it via a microcontroller.

    While I could pull the one off the wall, I patiently waited for my test unit to arrive.

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Trueman wrote 05/30/2018 at 16:37 point

It's a great job of Reverse Engineering

I have a very similar device but the communication parameters are 500 Baud, 1 Start bit, 8 Bit of data, Even Parity, 1 Stop Bit.

At the moment I am developing the software for Arduino and I am going to adapt a circuit of a Lin interface for cars.

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Glen Ogilvie wrote 02/12/2018 at 22:16 point

Any update on this project?    I'm about to buy an air conditioning unit (ducted) and want to be able to control it from a raspberry pi / esp8266 / esp32.

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Neil wrote 02/08/2018 at 05:23 point

I've got a similarish unit and have discovered the CoolLink range of adapters to communicate with them. Not sure of cost at the moment.

Interestingly, in the documentation for their latest units:

It mentions the interface to Fujitsu heaters is through a Echelon U10 interface, which is LONWorks. I can't find many reference to LONWorks with Fujitsu, except using the "network converter", so not quite sure how this works.

I have an RS485 adapter too, so I could also see if that picks up anything on the remote connection...

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James wrote 01/26/2018 at 06:29 point

The documentation from Intesis interface used by @Beau McMurray contains details of the error codes - which could be very handy. What are the chances that the other details of the modbus protocol/registers in the document are 1:1 with the addresses of the Fujitsu native serial commands?

Documentation here:

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Beau McMurray wrote 01/24/2018 at 08:27 point

@johntamplin @Myles Eftos Not sure if you had much success with this, however I can confirm the Intesis Modbus unit (FJ-RC-MBS-1) works beautifully as an interface with Fujitsu systems using this remote control. 

I'm controlling mine now via a Raspberry Pi using Node-RED, with plans to turn it into a fully automated zoned system.

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Myles Eftos wrote 01/24/2018 at 10:24 point

interesting! According to my 20 second google is uses RS485 for the bus. How much was it and where did you get yours?

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Beau McMurray wrote 01/28/2018 at 12:59 point

It cost me $396AUD including shipping from IvoryEgg. Worth every cent IMO for the ease of use. Got it up and running in no time!

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Myles Eftos wrote 01/24/2018 at 10:30 point

ah. This converts their protocol into RS485.

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Beau McMurray wrote 01/28/2018 at 13:00 point


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Glen Ogilvie wrote 02/12/2018 at 22:19 point

Hi Beau,

Could you provide a few more details as to how you have your raspberry pi talking to the FJ-RC-MBS-1 ?

Did you get a modbus shield for the raspberry pi?
wondering if this is the way to go, or if we are so close we can control these units without it?

Can you share the code you are using to talk to the modbus interface?


Glen Ogilvie

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johancoppens wrote 01/22/2018 at 12:09 point

Have you succeeded in a working installation? I want to control the heaters installed at our school from an arduino. Can you confirm that it is LIN protocol? I have ordered a MCP2025 to test it with an arduino. We will see what the results will be

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Myles Eftos wrote 01/24/2018 at 10:26 point

it’s been suggested it might be RS485. I will try and find an interface and see if that does the trick

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Andrius wrote 12/27/2017 at 10:06 point

Subscribed.  My house is heated/cooled by similar 10kW Fujitsu-General duct unit and I'm looking for it's better automation than gives this legacy control panel, a something IP based control. Are you aware that this wired unit might be replaced with IR control? And that IR unit might be controlled by on the market avialable universal IR/wifi/home automation devices? IR device part number is UTY-LRH. Also worth mentioning and it might be helpfull searching for information that the same AC/heat pump units manufactured by Fujitsu were sold under multiple brand names- Fujitsu, Fujitsu-General anf Fuji Electric and might be even more.

The unit might be controlled by multiple remote controls- wired and IR on the same wire.

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adam.pang wrote 10/25/2017 at 07:04 point

Hi @Myles Eftos, it's good to know you are doing a similar project as mine. I am also using ESP8266. I wished to control my office aircon through IR technology but I noticed it's actually using a wired remote control. Have you succeeded? Would like to hear from you soon if you have figured it out....

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bullety.hasor wrote 08/16/2017 at 17:48 point

Really good work @Myles Eftos !!! Here another person interested in your project. I've same unit at home and I'd like to control it with ESP8266. Any new clue about protocol involved in communication? Maybe LIN protocol as @Eddi suggested?   Thank for your time publishing the project here.  

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johntamplin wrote 07/25/2017 at 08:40 point

This is EXACTLY what I've been trying to achieve; same unit, same damn Fujitsu unit. I did discover a KNX-based interface, but the costs involved there are prohibitive. How has progress been since your last post/log ?

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rlopezcarreras wrote 07/03/2017 at 16:01 point

Hi all! Great work! Have you made any progress on this? I'd like to implement the protocol in a raspberry with a LCD for controlling the AC... it seems not easy... thanks!

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Myles Eftos wrote 03/13/2017 at 09:04 point

Thanks @Eddi - I'll have a read up and see if you are right.

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Eddi wrote 02/10/2017 at 19:32 point

after reading i suspect you may have lin protocol as well... the 3 spike ad highier frequency may be a sync

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