Initial information for this project came from Paul’s blog post from 2020 here:

https://c65gs.blogspot.com/2018/12/super-simple-protovision-compatible.html

Recently (2022), Gurce tried to create the same joystick expander for himself, gleaning details from the blog post and also diving into more of the details, covered in these discord threads:

How to create the adaptor

So in brief, I’ll share how I went about creating the adaptor and how to make use of it.

I purchased these handy experimenting/hobby-friendly cartridge-port pcb boards and two male DB9 connectors:

https://www.ebay.com.au/itm/401226011480

Here’s a diagram of how I wired them up:

Connect /DMA to GND pin
Connect the PCB’s JOY3 signals (up/down/left/right/fire/gnd) to JOY3 DB9
Connect the PCB’s JOY4 signals (up/down/left/right/fire/gnd) to JOY4 DB9

NOTE1: Recent bitstreams disable the adaptor by default. To enable it, you need to type:

POKE $7010001, 14

NOTE2: Older bitstreams experienced a ‘sticky movement' issue on joystick #3. To resolve this, assure you have the following bitstream or newer (assess from git hash or date):

commit c0ab2a448a97131edbf0148251d96d3fdd2c878d
Author: Gurce Isikyildiz <Gurce.Isikyildiz@resmed.com.au>
Date:   Mon Aug 1 08:22:54 2022 +1000

    minor tweak to fix joystick 3's 'sticky' movement issue

NOTE3: This adaptor board is only intended for use with the MEGA65.

Do not use it on your Commodore 64, it won’t work there!
If you need such an adaptor for your Commodore 64, purchase the following adaptor from protovision (to be plugged into the user port):
- https://www.protovision.games/hardw/4_player.php?language=en

Happy Snaps

Here’s some happy-snaps of it all connected up:

MEGA65 Wiki

Super simple Protovision-compatible Joystick Expander for MEGA65

How to create the adaptor

Happy Snaps