Print from SD and connect to serial host at the same time I am wondering if it is possible to print with a 3D printer from an SD card and connect to a serial host at the same time. I want to use the Marlin M118 serial print command to trigger custom functions running on a Raspberry Pi. The RPi would be connected to the printer over serial but would not be sending G-code. Is this possible?

My printer is the Monoprice Mini Delta. It runs a Marlin based firmware and I can install standard Marlin on it if necessary.
This can be done, but you need to have the right order of operations. Octoprint relies on such a setup. 


Set up the serial connection first, as sending the connect signal from your terminal or Raspberry to Marlin triggers the printer to reboot.
After having the connection established, start the printjob either via the control buttons or the remote console (for example: Octoprint or Repetier Server, running on your Raspberry)


Note that you have to make sure not to dis- and reconnect the raspberry during print, as that might trigger a reboot of the printer!