Arduino Due Programming vs Native Port + No I2C Detection

Having trouble detecting I2C devices at any address. I2C scan returns a found address for an I2C device but then in future iterations of the scan, it reverts back to "No devices found" which brings concern for some inconsistency in the connection.

I am using an Arduino Due that is connected to my computer. The Due is also connected via SCL and SDA to a DAQ device. Does connecting the micro-USB cable to the Programming Port instead of the Native Port affect the I2C detection at all?

Would anyone happen to know a solution for I2C detection?

Please post a schematic and a link to the the DAQ device..

Do you have any external pull-up resistors to 3.3V on the SDA/SCL lines? Worth trying those. 3K3 or 4K7 should be ok.

Not related.

Yes, just use one of the standard I2C scanners, the Multispeed one liked to above works for instance.

I have maybe 3 DUEs, no problems with I2C scanners or I2C devices on any of them.

No I2C addresses are found. But when I toggle the address range from 0..127 to 8..120, I get a V in the column with the header 200. What would this imply?

Its found a device at the address given.

Note that the 'headers' are labelled [KHz], so the devce was found when the I2C clock was 200KHz.

It found 19 devices...is this unexpected behaviour? And if I rerun the scan, the addresses change by 1.

And this happens when there are no I2C devices connected ?

I have the DAQ device, sensors, connectors, cables all connected. I should expect an I2C device to be found, but the scan returns inconsistent results. Now, no I2C devices are found anymore. What could this problem be related to??

As asked in #9 what happens "when there are no I2C devices connected ?"

When no I2C devices are connected (SCL and SDA disconnected, no jumper wires on the Due connected), the scan returns no I2C devices found at any address

Find a different I2C device to test.

Could my custom PCB boards, the DAQ device or Arduino Due be damaged? If I were to use an Arduino Uno instead, will I need pull-up resistors? My current board system runs on 3.3V

It's not detecting a I2C device even with another I2C device connected. I've connected the Adafruit LIS3DH Triple-Axis Accelerometer to a breadboard with 4 connections (Vin, GND, SDA, SCL) and I2C detection fails.

(https://www.digikey.com/catalog/en/partgroup/lis3dh-triple-axis-accelerometer-2g-4g-8g-16g/57890)

Ah ..... a custom PCB is it, useful to know ...........

If the DUE with nothing else connected apart from the LIS3DH sees nothing on I2C then either you have it wired wrong, the DUE is faulty or the LIS3DH is faulty.