This article describes connecting a WIZnet W5500 Ethernet controller to Sega Dreamcast and NAOMI/NAOMI 2 systems using existing serial interfaces configured for SPI. The W5500 is used in MACRAW mode and provides raw Ethernet frame access. The driver has already been merged into KallistiOS master and will be supported in the next version of DreamShell.
Here are the tests through the network stack when copying a file via FTP:

• SCIF SPI: TX ~1000 KB/s (PIO), RX ~550 KB/s (PIO)
• SCI SPI: TX ~900 KB/s (DMA), RX ~1000 KB/s (partial DMA)

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Dreamcast — SCIF

This variant uses the SCIF serial port.
Signal mapping follows the standard Dreamcast serial connector pinout.

SPI Signal	DC Serial Pin	SCIF Function
MISO	RX	SCIF_RXD
MOSI	TX	SCIF_TXD
CLK	CTS	SCIF_CTS
CS	RTS	SCIF_RTS

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Dreamcast — SCI

This variant uses the SCI interface.
Signals are taken directly from resistor pads on the Dreamcast motherboard.

SPI Signal	Motherboard Pad
MISO	R115
MOSI	R122
CLK	R140
CS	RA101

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NAOMI — SCIF

On NAOMI and NAOMI 2, SCIF and SCI signals are available directly on the CN1 connector.
No motherboard soldering is required.

SPI Signal	CN1 Pin	SH-4 Function
MISO	B45	SH4_RXD
MOSI	A46	SH4_MDI7 / TXD
CLK	A47	SH4_MD07 / SCK
CS	B50	SH4_MD8 / RTS2

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NAOMI — SCI

This variant uses the SCI interface available on the CN1 connector.

SPI Signal	CN1 Pin	SH-4 Function
MISO	B45	SH4_RXD
MOSI	A46	SH4_TXD
CLK	A47	SH4_SCK
CS	B50	GPIO / RTS