I came across a very clean PlayStation Net Yaroze Serial to USB adapter post.

[JakobAir]

It's at https://delta-island.com/index.php/en/tutorials/stu-net-yaroze

The write up is straight forward and involves the usual use of DosBox and links to download the PlayStation side of things. The only issue right now is that I can't find a link to actually buy the adapter.

 

[torrent_get]

This post is 4 years and 2 days old but the user had a small number of units available.

https://www.obsolete-tears.com/forum/viewtopic.php?t=8788

 

[JakobAir]

This post is 4 years and 2 days old but the user had a small number of units available.

https://www.obsolete-tears.com/forum/viewtopic.php?t=8788

Well that's disheartening that they're all sold. I messaged him on eBay a few hours ago to see if he's got any left.

 

[master801]

Looks like a pretty simple adapter that anyone can design and make on their own.

Serial port harvested from a PS1 and some header pins for the TTL serial USB adapter. Not sure what the resistor and diode spots are for though.

 

[SylverReZ]

You can wire the connections from the FTDI adapter to the test points for the SIO port on the motherboard if you don't have a spare serial cable to sacrifice.