Currently, my OctoPrint RPI will partially power the RAMPS 1.4 unit over USB for my printer, causing the LCD to power on.
I'd like to set things up so that the RAMPS unit only powers up when it's being supplied +12V from the printer's power supply.
Is there a tried and true method for this, such as a setting in Octoprint or cutting the +5V wire on the USB cable connecting the RPI to RAMPS?
It’s nothing to do with OctoPrint software, it’s the Raspberry Pi. You could use a data only USB cable, a usb power blocker (a guy sells them on Tindie and TH3D should carry them, or remove the USB —> 5v diode from the RAMPS.
I may have seen somewhere that it’s possible to configure newer RPI’s to not provide power on any USB ports, but I wouldn’t mess with that.
I will note that we've discussed this many times on here so please search the forum for "5V", "backpower" and such.
I'm starting with the why part: If you connect a 3D printer via USB to your Raspberry Pi it can lead to all sorts of power related issues.
This statement is too generic and therefore false. Not all 3D printer controller boards draw power from the USB connector. Only those that do may require that something is done to isolate them from the RPi.
This is partially due to the fact that the normal RPi power input isn't sufficient to provide the USB spec of .5 amps to USB 2.0 (or .9 amps to USB 3.0) ports.
I used some electrical tape to cover the pin, thanks all!