The here shown C64 User-Port Expander is a recreation of the Scanntronik Userportweiche from 1990.
This Expander extends the C64 with 2 additional User-Port slots. The electronic logic switches automatically between the connected devices. The circuit contains a driver IC to protect the C64 from overstressing its port IC and to provide enough power to connect long cables. One slot is especially dedicated to connect a printer.
Last Update: 25.08.2019 
C64 Userport Expander (rebuild)
The original and the rebuild.
Boards in the making.
The schematics (v1.1 / v1.2)
Layout v1.2
v1.2 - ground planes
Top Silkmask v1.2
Bottom Silkmask v1.2
The new design (v1.2).
It contains all three parts of the board.
How to build the expander.
The adapter can be bought soon as an assembly kit at

(I'm not associated with in any form. I've only ceded the design of the board for use to him.)
3D Rendering Top
3D Rendering Bottom
With Components