Nold's Alternative Dreamcast Pinouts & Workarounds
I took some time off and took a deeper look into the dreamcast. Here are the results!
VGA- & Composite-Video, Audio & SD-Card wiring
Nothing special here... The filters for R, G, B, composite (and luma/chroma) can even be removed. Got no problems with a long VGA-Cable. (H/V = Sync for VGA)
Note: You'll still need some additional components for VGA! See f.e. mmmonkey's website. (100uF caps also work fine).
Faking the FANs PWM-Signal
Why use Arduinos or 555's... damn... just use some solder!
Some background: The DC uses IC601 (a multivibrator IC) as some kind of ADC (Analog-Digital-Converter), to convert the PWM-Signal from the fan (up to 2.5v), to 3.3v (= logical "true"). So when there's no fan, it'll put out 0v (= logical "false"). So all we need to do is pull-up the output signal of the IC & done! How nice Sega prepared this mod for us & gave us some free solder-pads.. (It might be safer to use a resistor here, but it should even work without the IC, anyway; also there is a capacitor, protecting a possibly connected fan.)
"Faking" the GD-ROM Clock / AIDA-Clock Signal
Last I've found out that the Dreamcast has an internal PLL, that the workaround shown in this post uses and it actually works! (With SWAT's DreamShell no g1-ata BIOS flashed)
Background: As told on the image, the AICA (Dreamcast's Audio-Chip) needs a clock signal, which it'll use for it's RAM-BUS-Clock (2xCD_CLK). The GD-ROM normally puts out a 33.8688MHz, so RAM-Clock is 67.7MHz; Without a GD-ROM audio will be funny sloooowww.... :D but it'll also slow down the DC.
So I've found out (using the available Dreamcast schematics) that the DC has an internal PLL which generates a 33MHz clock. And the Test-Point in the image is the best connection point for it!
So this will basically slow down the AICA RAM a little bit, but I played for a while & couldn't notice any difference.. otherwise just use a active crystal...
Hope these images are of any use for someone! :)
//Update 5.12.17: Cut trace for PWM-Hack to prevent current sinking (26mA) through IC601.
The original article is available at: https://bitbuilt.net/forums/index.php?threads/nolds-alternative-dreamcast-pinouts-workarounds.1822/
