SukkoPera 16UP

16UP is an Open Hardware Expansion board that will add a User Port to the Commodore 16, 116 and Plus/4 home computers.

Source: github.com/SukkoPera/16up/tree/v3

SukkoPera Plus4Serial

Plus4Serial is an Open Hardware Serial Port Adapter that plugs into the Commodore Plus/4 User Port, turning it into a fully-functional RS-232 interface.

Source: github.com/SukkoPera/Plus4Serial