Code and configurations to use LinuxCNC to control a RepRap 3D printer
Copy the contents of the components/ directory to src/hal/components in a LinuxCNC source working directory and build as usual.
Run stepconf to generate a configuration for your specific 3D printer, or start with the MendelMax.hal or MendelMax.ini file provided.
Copy the custom* files to your LinuxCNC configuration directory to load the I2C ADC driver and add an extruder temperature monitor panel to the axis gui.
The interface between a standard PC parallel port and the RAMPS board is pretty simple (see KiCAD schematic and PDF file in lcnc2ramps).
There is a single transistor inverter to convert an output pin into an open-collector drive SDA signal. Combined with a second output for SCL and an input to read the SDA status these three pins form an I2C bus to talk to the ADC (and anything else you want to connect and write code for). If you do want to add additional I2C chips, crafting a command is reasonably straight-forward. E-mail me if you can't figure it out from the comments in the I2C.comp file.
There are two more single transistor inverters to insure the RAMPS FETs have enough gate drive to turn on hard.
Finally, the RAMPS board is powered by 3.3V and NOT 5V. This is because of the ADC input requirements and to insure the stepper driver chips see the 3.3V high typical of most PC parallel ports as a logic high.