A simple hardware modification to disconnect the JVS controller from port 0 (port A on the Dreamcast). You just need solder jumper JP3 and switch it. This is a very useful for DreamShell and running Dreamcast games by it.
After this, you can connect all Dreamcast peripherals to port A. This will result in all four ports available on the side board: A, B, C and D.

You need to find it on your revision of motherboard. The jumper pads is present on all NAOMI revisions. Here I will show an example on the most common version:

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This guide explains how to access the SH-4 SCI port on Sega Dreamcast and NAOMI systems in both UART/SPI communication modes. It can be used for debugging or connecting an SD card. In SPI-like mode, the SCI port can transfer data at speeds up to 1.5 MB/s, which is twice as fast as using the SCIF port for bit-bang SPI.

Sega Dreamcast Motherboard

Solder wires directly to the following points on the Dreamcast motherboard:

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Серийный порт

b1 +5V
b2
b3 GND
b4 RX2
b5 TX2
b6 RTS
b7 CTS
b8
b9
b10 +3,3V

Параллельный и GD-ROM порты