RS-232 Serial Cable

The WonderSwan extension port supports 9600 and 38400 baud UART transfers - it is used for connecting two consoles together, as well as peripherals like the MobileWonderGate. An official PC adapter was produced by Bandai and bundled with the WonderWitch; however, it is difficult to find today.

RS-232 serial adapters

Some serial adapters connect to a computer via a DB9 (RS-232) cable.

DIY projects

MAX3232 adapter

Junkbox.info provides a tutorial and schematic in Japanese on assembling an RS-232 adapter using a scavenged extension port connector and an MAX3232 or compatible chip.

USB serial adapters

Other serial adapters connect to a computer via USB directly.

Products

RetroOnyx USB Link Cable

The RetroOnyx USB Link Cable is a ready-made USB serial adapter. Notably, it uses a proper replica of the extension port connector, as opposed to repurposing an HDMI port.

DIY projects

FTDI TTL-232R-3V3 adapter

Zerosquare has written a tutorial in French on how to assemble the adapter using a scavenged extension port connector and an FTDI TTL-232R-3V3 cable.

ExtFriend

The ExtFriend is an Raspberry Pi Pico (RP2040) firmware which implements USB serial port adapter functionality. In addition, it can optionally act as a headphone adapter for USB audio capture.

Port Zero has written a tutorial in Japanese on how to assemble the adapter using an HDMI connector.