I am having issues with a PR33-28 board from NCD. It does not show up on the list of i2c addresses but the PR20-10 does show up and a DS1307 clock I had laying around shows up.
Hi @jsmedley,
I’ve used some of the NCD boards before (not explicitly the PR33-28 however). I was wondering, how have you connected the I2C bus from the Fin to the PR33-28? You may need to check the pull-up resistor header on the PR33-28 (see the image below) if you aren’t using any NCD adapters.
If you could send a picture of how you’ve connected the Fin to the PR33-28, I might be able to help!
Cheers,
Alex
Interestingly I was getting away without a shared common ground and with a shared common ground everything works now.
Correction: Our MC7455 is causing ths not to work when it is connected all i2c channels respond.
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Glad to know you got it working! 
I have confirmed that the Project Fin board has issues with i2c communication with 2 different modems attached.
MC7455 and SIM7100
Hmm this is curious, I’ll check this out tomorrow and see if I also have similar issues with my ME909s-120. If you run the i2cdetect -y 1 via apt-get install -y i2c-tools do you see any devices listed? I assume that it works without the modems attached?
Yes I see the 5 address I expect respond back but then once the modem is installed all address respond back when running i2cdetect -y 1 and it works without the modem installed.
It would be interested to know if you have the same issue.