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I have a 40 pin ribbon which connects an RPi 3 B (SPI and GPIO) to another microprocessor. It seems to work fine on a 3B. I've recently found out that if I connect a 3 B+ instead the Pi will not boot, solid red power, no activity LED.

Additionally, if I boot the Pi 3 B+ without the ribbon connected, powered by USB supply, then plug ribbon in after boot, it works fine. If the ribbon is attached AT boot time, then it will not boot.

Anyone know a potential cause? I couldn't find anything with a search. Everyone states pinout is the same between 3B and 3B+. Is there a difference in initial GPIO config that could cause a GPIO to be in output mode on boot?

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power issue? 3b+ draws more than 3B

  • That was my first thought, but I am powering it with supply the B+ came with and it boots fine if the ribbon is not connected. I also read if power is no good on B+ the red LED should blink, mine is just solid. – Dom G Sep 23 at 18:21
  • As additional detail, if I boot the Pi 3 B+ without the ribbon connected, powered by USB supply, then plug ribbon in after boot, it works fine. If the ribbon is attached AT boot time, then it will not boot. – Dom G Sep 23 at 18:25
  • You are right about the red LED. Solid red light indicates sufficient voltage. – yesno Sep 23 at 18:34
  • Here is a whole thread on the issue. Quite a number of different things can cause this. – yesno Sep 23 at 18:42
  • I did read that before posting, but none of it seems to cover this case, where I can boot without anything connected to GPIOs/SPI but can't boot with something connected. The only thing I can figure is something in the boot sequence inside the VideoCore boot CPU code is checking something and halting. If I don't plug the cable in till after boot everything works fine. – Dom G Sep 23 at 20:44

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