I'm currently dealing with a circuit for communication between a Microcontroller and an e-ink Display. It uses an SPI-Bus. The display is from PervasiveDisplays, and at the bottom of this website http://www.pervasivedisplays.com/kits/ext_kit there is a download containing gerber files for the kit including the circuit for the extension board. The part I'm struggling with is the following:
To me this is a mosfet switch that conducts the microprocessor signal when the gate-source voltage is positive. The problem I'm having is that this input is connected to the drain. If it was connected to source (Drain being output), I could see the MOSFET conduct and put the drain pin on a low-voltage if the source is low, whereas if it was high, it would be pulled to 5V by the resistor. However, it's the other way around and I can't really wrap my head around how this part of the circuit works. Any help is greatly appreciated.