Guide Nold's Alternate Dreamcast Pinouts & Workarounds

Nold

I just meme here
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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).

dc_alternative_vga.png



Controllers & RTC
Every pair of filters is for one controller. They are connected to pin 1 & 5 of the controller. DON'T remove these filteres! (There are connections on both sides of them).
The RTC voltage is connected to pin 2 of the connector, using a 10Ohms 1W resistor. Don't use a 3.3v Battery here! (The Dreamcast uses recharable batteries).
Last we have the PWM-Signal comming from the FAN. It's required for the DC to boot, but we won't need it...
dc_alternative_controller.png


Faking the FANs PWM-Signal
Why use Arduinos or 555's... damn... just use some solder!
fake_pwm_hack.png

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)

dc_no_g1ata_clock.png

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.
 
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Shank

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This is great! Dreamcast needs some love. If you aren't already, hop on the discord to the Dreamcast initiative channel and we can compare notes.
 
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This is an amazing guide great work dude I will be using this if I ever find some time to work on my Dreamcast portable
 

Nold

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are alternative pinouts the same as alternative facts

... and we're gonna build a HUGE portable with that! And I'm gonna make ShockSlayer pay for it! ;-)
 
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