This news update is long overdue, but there are reasons for that.
Back in 2015, @marvin96 posted a story over on my website EurAsia titled PsNee, an open source Arduino based stealth modchip for PSX (Sony Playstation 1). The project development progressed over at assemblergames.com until the website was shut down for several months in 2017, during that downtime I forgot about the project.
But fortunately the developers didn't and moved the project to psxdev.net, and now there is also a PsNee Project at GitHub. There has been a lot of progress and this impressive open source modchip is now working stable according to several reports. The source code is multi-region, meaning it will unlock PAL, NTSC-U and NTSC-J machines, handles antimod games while remaining stealth, it supports Arduino ATmega based boards and ATtiny MCU.
For those interested in the in-depth technical details I suggest taking a look at the Arduino source code. There are plenty of comments explaining the different patch stages for disc auth and how the BIOS patching is done to support NTSC 60Hz video signal on PAL PM-41 boards (PSone). The BIOS patch is waiting for address line 18 to go high, then it patches by holding data line d2 low. A clever minimalistic solution which means less intrusive mod with less wires.
This is what makes open source so much fun, contagious development where everyone is eager to improve and help out.
Project home: --> https://github.com/kalymos/PsNee