D-Bus pull-up resistor investigation

[hn]

I have conducted a more detailed investigation into the pull-up resistor behavior of various D-Bus components to ensure signal integrity across different hardware setups.

Methodology

To determine which components feature internal pull-ups, I tested several modules individually (without other bus participants). Each module was connected to power, and the DATA pin was tied to GND via a "weak" 50kΩ resistor. I then monitored the DATA line level using a logic analyzer to see if the module would pull the line high.

Test Results

The findings across the tested modules are as follows:

Component	Observation	Conclusion
Unbalance Sensor	Pulls DATA high upon power-up.	Internal Pull-up (confirmed at 10kΩ).
Timelight Module	Pulls DATA high upon power-up.	Internal Pull-up present, confirmed 10kΩ here
User Interface (EPW66027)	Pulls DATA high upon power-up.	Internal Pull-up present.
COM1 Module (8001056347 & 8001056350)	No voltage rise on DATA.	No Internal Pull-up.

Note: Both COM1 variants were tested; despite different part numbers, they appear functionally identical regarding the bus interface.

Analysis

It is currently unclear why most bus participants include a pull-up while the COM1 module does not. Generally, pull-up resistors are highly recommended for components connected via longer cables, especially in environments with high electromagnetic interference (EMI) from motors or other appliance electronics.

The COM1 module is known to be used in two primary configurations:

1. Long-run: Connected to the Power Module via a longer cable (e.g., in dishwashers).
2. Short-run: Connected directly to the User Interface module via a short cable (e.g. in a dryer)

While a pull-up would be expected for the long-run scenario, the COM1 seems to rely on other bus participants to provide the bias.

@LiamHD is having trouble connecting the adapter to a dishwasher control panel; this could be due to the pull-up resistance being too high.

Recommendation

While I am not an EE expert, I believe that to ensure maximum compatibility and reliability, the D-Bus adapter should feature a switchable 10kΩ pull-up resistor (via jumper, switch, or solder bridge).

This allows the hardware to adapt to different bus loads and cable lengths. To my knowledge, many adapters for other bus systems (such as I2C, Modbus, CAN, etc.) offer a switch to optionally add a resistor depending on the scenario, so this may be considered best practice.

@Bouni @kiu @tubist2004 @LiamHD

[LiamHD]

@LiamHD is having trouble connecting the adapter to a dishwasher control panel; this could be due to the pull-up resistance being too high.

I agree. See: #58 (reply in thread)