A construction manual can also be found on circuit-board.de . This Multijoy interface originally comes from Radek Sterba .
In the interface, a decoder is controlled with 3 input pins, which switches the ground connection of the selected joystick, so that the pressed direction or the fire button is then grounded and can thus be queried via 5 output pins. All joysticks can then be read in one after the other in a loop.
The Multijoy interface can be connected to the C64 in two ways:
direct connection to the joystick ports
This variant is supported by Mashed Turtles V2 , for example
 
Connection via a user port adapter
This variant is supported by the first version of Mashed Turtles