about dreamcast's serial-port

[SWAT]

In normal mode, with internal clock, max speed is 115200 baud.
In normal mode, with external clock, max speed up to 3 mbaud, however unlikely it will work well at this speed, I think max 1.5 mbaud.
In custom mode (SPI emulation), speed depends of CPU. If CPU idle, speed can be up to 500 kbytes per second, but CPU is usually busy with other calculations, especially in games.
This port is officially used to connect the two consoles for multiplayer game.