Dreamcast Serial RS-232(C) Transmission

Dreamcast Serial RS-232(C) Transmission:
Getting a Dreamcast to communicate with a PC via RS-232 serial port

-Suggested Prereading: RS-232 Technology Basics and Overview

Introduction:
About the Dreamcast SCI
The Dreamcast's serial communications interface is one of the SH-4's (Main Processor) possible serial connections. The bus was rarely implemented and was often used for obscure purposes like the Dreamcast MIDI cable which was compatible with only 1 Japanese Dreamcast game, or the Dreamcast to Neo-Geo Pocket Color Link cable for a handeld device to interact with software running on the Dreamcast. The most common use for the Dreamcast's SCI port was for Dreamcast to Dreamcast Communication in the way of two systems running the same game on seperate displays so 2 or more users/players could run software (games) on two seperate displays, each system transfering user/player statistics back and forth via the "Dreamcast Versus Cable".

The Concept:
Instead of using the Dreamcast's serial port to interface with a device or another Dreamcast, we will attempt to use this port for Dreamcast to PC communication, in the hopes that we may download and upload code to and from the Dreamcast to and from a PC. See the simple diagram below:

The box connecting the Dreamcast to the PC represents a circuit that will convert the voltage levels of the Dreamcast and PC so they will be compatible. This will be the main focus for the Dreamcast Serial Interface.

Below is a physical representation of what we should expend the end result of our DC<->PC Serial Interface. We see a proprietary Dreamcast serial port connector with its exopsed leads going into the Transceiver/Level Shifter circuit, a 2 meter long db9 serial (RS232) cable is attatched the the circuit from the box, it can be removed for convenience. The only real difference we should expect to see for our REAL final result is that the wires coming from the Dreamcast will not be exposed, also perhaps indicator LEDs and other small additions will be made.

NOTE: This design is the default style for DC<->PC serial adapters, however many things in this circuit can be changed such as the methods you use to obtain the Dreamcast and PC RS232 connections, the length of the cable you decide to used, the size and location of the level shifting / transceiver circuit.

CABLE LEGNTHS:
If you are considering having a lot of space between your Dreamcast and your PC's serial port, you may want to figure out a way to get your Dreamcast closer to your PC or vice versa. If you end up using a lot of cable the capacitance of the wires in the serial cable will be large, this causes a noise/distortion/crosstalk and other headaches for someone trying to communicate serially (via RS-232).

Physical view of indivudal connectors

FOR THE OPTIMAL DESIGN:

If you wish to obtain data rates that many enjoy with "coder's cables" or DC<->PC Serial adapters, around 115,200bps, you should limit the total length of your cable to 3 meters (~10 feet) or lower. The slight differences between the Dreamcast clock and your UART's (PC serial port) clock make the serial connectivity "ify" and when you add other issues such as long cable length it can be quite difficult for you to get them both into sync. Since there are differences between the DC and PC serial interfaces concerning the clock, some devisions (slower data rates) may not work. 115,200bps , 57600bps , 9600bps are common data rates used when cummincating between a PC and Dreamcast using this transceiver adapter. I find that 57600bps tends to be the most reliable baud rate, so make sure to test this speed if you cannot get others to work.