Need help to find out which protocol is used - reverse engineering joystick

[j0q]

Hi all,

I'm currently trying to upgrade my Logitech G940 Throttle.
It was used as a flight system together with G940 Pedals and G940 FFB Joystick.
The pedals and throttle would plug into the joystick via 9-pin (RS 232) connectors. The joystick would then be connected to the PC via USB.
Because I don't need the Joystick and Pedals anymore (designed/modified and 3D-printed my own), I'd like to be able to use the throttle directly via USB. Therefore, I'm trying to read the throttle outputs (potentiometer values and switches) via an Arduino Pro Micro. There should also be a possibility to set the status of the onboard LEDs of the throttle (they illuminate the buttons on the throttle base) later on after understanding and fixing the communication.

Only 4 pins are used for the connection between throttle and joystick.
2 wires are used for +5V and ground. 1 wire transfers information as you can see on the screenshots and one wire is completely dead (= 0V, at least on the throttle side).

There's plenty of information about the connection between pedals and joystick (pinout diagrams etc). Unfortunately, I couldn't find a single post about the connection type / wiring diagram between throttle and joystick. All the modifications that are published in various forums rely on the joystick's USB connection.

I've done a little bit of research about the hardware. The chip inside of the throttle is called Holtek ht46ru232 - the datasheet said that it uses I2C as communication protocol.

However, the signal graphs captured by the oscilloscope don't look similiar to I2C communication (used ADS1115 for comparison, see attachment) and a short test script wouldn't output anything.
The voltage on the active signal-wire of the throttle drops significantly to less than 0.5 V if connected to SDA/SCL pins

Attachements:
2 screenshots of ADS1115 communication via SDA & SCL (I don't know which belongs to which)
2 screenshots of the same signal wire of the throttle, different capture times (there's difference in the signal, maybe that's a hint to somebody)

Do you have a clue how the communication works and if there's a possibility to use an arduino?

Thanks in advance,
j0q

[Koepel]

I2C_ADS1115_line1.png is the clock signal SCL. You can count 9 clock pulses for each byte.
I2C_ADS1115_line2.png is the data signal SDA.
I have no clue at all about the first two.

[gilshultz]

The waveforms would be much easier to figure out with a logic analyzer. A 24 MHz version can be gotten from your favorite china supplier for less then $10.00. They with the proper software will even give you the data that is transpiring across the I2C or most any other protocol.

[j0q]

Thanks Koepel and gilshultz for your replies. I'll have a look at the logic analyzer.