I just built the backpack and works great, thank you!
Is it safe to leave the FTDI serial cable connected to an USB port when the display is powered and running?
I have noticed that if I plug the USB the backpack *and the display* are powered from the usb port (yuck!)
You can leave the FTDI USB cable connected to the backpack+display ONLY if you are providing 5V power from the power adapter. You can’t power the backpack+display with USB alone (as you found). Make sense?
Um, yes, I was more concerned that in that case, am I not basically powering the USB port, and possibly the entire +5V inside the host, which in my case would be a raspberry pi, plus its own power supply, with a non current limited source? (I don’t think there’s a diode or a pptc, fuse or any kind of protection anywhere, i am just putting the two supplies in parallel, right?)
The USB port on the Raspberry Pi is at 5V. When you connect a power source to the backpack using a power adapter, that also sets the 5V line on the FTDI cable to 5V. So both the Pi and the backpack are setting this line at 5V, which is fine. No current flows because they are at the same voltage. If you unplug the Raspberry Pi, then there could be current flowing into the Pi, depending on the design of the USB on the Pi, but it probably has a diode in there somewhere. Having all this connected and powered at the same time is fine, but don’t remove power from either the backpack or the Pi.