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?