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 games by it.
After this, you can connect all Dreamcast peripherals to port A. This will result in three ports available on the side board: A, B, and C. Port D is not routed on it, but may be available on the motherboard connector.

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 for both UART/SPI communication modes. This is can be used for alternative SD card modding or debugging. In SPI-like mode, SCI can reach transfer speeds of up to 1.5 MB/s, which is x2 faster than mod 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