This is a 3-5 volt compatible programmer dongle which attachs to the parallel port. Compatible with UISP.
The four line used to upload the code, MOSI, MISO, SCK and /RESET have all diode protection and slw rate limiters. This all makes it possible to use a long cable between the project and the desktop.
The PC parallel port side has a simple level shifter which simply makes the signal to bang it's head to the limiting diodes as needed. There is a transistor to keep the VCC from climbing up with the input signal.
Lasttly there is a HC125 to buffer the four signals and to switch them off as needed.