I'm trying to use one of the RAMPS GPIOs to control an external device that requires a 5V low-current logic level signal from Marlin. In order to do this programmatically, my host software (Octoprint) is sending an M42 command. I am using the following syntax:
M42 P4 S255
according to the pinout in the following image:
However, the pin appears to not be driven to a logic HIGH level. Is there firmware-level configuration I need to do as well, or is my syntax/pin number incorrect?