Hmm, seems like something is awry in fixed input 1 then. If your trains are working, that’s good, but the TIU should work fine with power applied to that input.
I have a Rev. I3a TIU sans USB therefore there is no power coming from the USB so I can’t test this, but maybe it’s a sequencing problem. For example maybe if there is power applied to the USB port before power to fixed input 1 it then fails to use that input for power?
Beats me. 🤷
I had a Stat professor back in college named Dr. Kelly. At one point, when I was struggling with a statistical “tree” Doc Kelly told me - “Mr. W, you need to simply accept that some things are and quit worrying about why they are.”
It was advice that served me well in Statistics from that point on and my overriding inclination right now is to simply leave well enough alone. 😉