I have one of these that is made specifically for the Pi. I am trying to use that as the primary display for the Pi. Currently, it is powered, but won't show anything on the screen. How do I get it to work?
there's no way to use this module as a primary display, it's intended for being accessed through the 4D Systems C Serial Library to draw things on the screen using commands sent over a serial interface. please, check the github repository on the page you get this module from.
you might be able to redirect some system output to the serial port and see it on the screen, provided your display module has "Serial Environment" software loaded inside.
Please read from the manufacturer's website:
Note: This 4D display will NOT function as the Primary Display for the Raspberry Pi. The Raspberry Pi OS can not be displayed on this display module, it is a slave/secondary display only. The display module is programmed using the Workshop4 IDE and the Raspberry Pi acts as the Host to send/receive information to/from the display module.
The statement "is made specifically for the Pi" means: the connectors interface easily with the Pi and there is software for it, but as the website says, you can not make it into the primary display.