Important notice: We will not be responsible for any damage to you, your PC, or your Dreamcast!
Buy a serial card that has a real 16C650 on it (not a compatible). The card also needs to have a GD75232 or compatible (near the serial port). To be fully compatible it also needs to be able to allow you to select Com8/$268, interrupts are not used.
We found cards that are marked "4022 Ultra RS-232" the easiest to modify.
16C650 cards have a 7.3728Mhz crystal on them (its a square silver thing with 4 pins on the bottom), remove this and replace it with a 25Mhz crystal. Be careful not to damage the board. The GD75232 needs to be removed (its a chip thats near the serial port, it has 20 pins). You will not need the GD75232 again, so don't worry about damaging it, your main concern is the board. After GD75232 has been removed you need to connect some wires where the chip was, connect:
2 to 19, 3 to 18, 4 to 17, 5 to 16, 6 to 15, 7 to 14, 8 to 13, 9 to 12
Whatever you do, do NOT connect 1 to 20 and/or 10 to 11!!!
Disconnect any jumpers for interrupts, they are not used. Make sure that the clock divider on the board is set so the crystal goes straight to the 16C650 (just make it so the board is in its fastest configuration).
Configure the port address jumpers to COM8 which is address $268...
CHECK ALL YOUR WORK TWICE BEFORE PLUGGING IN AND SWITCHING ON!!!
The 16C650 is only supposed to be driven a max rate of 24Mhz, we are driving it at 25Mhz (its being overclocked). After its been on for a while check that the 16C650 isn't getting too hot. If it is then fit some kind of heatsink.
DO NOT USE THE CARD FOR ANYTHING OTHER SERIAL COMMS AFTER THE MODIFICATION!!!