Original Xbox VGA Sync buffer pcb

[cyberosis]

Been looking into making a a small pcb to add proper V and H sync to the multiout on the original xbox. At first I was making one with the old design that used 4 transistors to amplify the signal from 1.5v to the proper 5v.

Spoiler

However, I found a post by Syntax where he used a IC designed to buffer the signal and amplify it.

viewtopic.php?f=6&t=61706

Looked like the better option so I started again. This time with the idea to lay the pcb straight over the multiout pins like this mod

Spoiler

So far I got the schematic laid out based on this resource Syntax shared.

Spoiler

I'm not 100% confident with it so I thought I'd share and see what everyone thought.

Spoiler

[cyberosis]

Went ahead and made a pcb with the schematic. The 74hct244 I decided to use is a tssop-20 which is the smallest one I could find. diodes are SOD-323F. The cap and resisters are 0605. Only the pins used are plated. There's a few grounds on the xbox video out, but I decided to use just the one label as (DC return) I was able to make the whole thing without any vias so there shouldn't be any change of shorting.

3D view

Spoiler

PCB

Spoiler

test fit

Spoiler

[Syntax]

Looks like it doesnt fit?

Try starting with the AVIP as a base, trim it down if you need too.

I made an all in one plug but it seems hotplugging it can spike and kill the GPU so i settled on an internal buffer like your design. Needs a sync combiner to make true csync now

https://www.xbmc4xbox.org.uk/forum/view ... b4139fe382

[cyberosis]

I did use the avip as a template, but you're right. it might be a little long. I'll do a few more test fits

C-sync sounds interesting. I could add a passive combining circuit like this one that's on retrorgb.

Spoiler

Would you drive it through the composite pin 23? would make the most sense since its used for scart mode. then you could use any off the shelf scart cable.

[Syntax]

Nah, do it properly

http://members.optusnet.com.au/eviltim/ascarten.png

[cyberosis]

Thanks for the help syntax! I found a tssop-14 version of that ic and was able to squeeze it in. Gonna share everything so far. Will probably get this one printed. Once I know it works, i'll share the files.

Schematic

Spoiler

PCB

Spoiler

Test fit

Spoiler

[maxtherabbit]

did you just pick two unused pins on the multi out for HV sync or is there some precedent for using those?

[cyberosis]

According to the GameSX pinout, these are the pins usually used for V and H sync in vga mode.

Spoiler

The original xbox never officially supported VGA, however my understanding is this pinout is based off the arcade version of the xbox called Chihiro. That version did have VGA out of the same multiout.

Spoiler

[cyberosis]

Did some small changes and Ordered the board. Time to test it!

[Syntax]

I'm unsure if c1 is required to be an electrolytic or ceramic. I never tested.

[maxtherabbit]

the sync buffer in the DC VGA box isn't the best. I would not recommend it for new designs

switch to the SN74LVC2G17 - it's what I use in my dongles and it's time tested in the OSSC

making that change would also allow you to omit most of the passives

[Syntax]

Nice suggestion.

I think the DC buffer was used with the intention to convert and buffer RGB to component, it has so many spare channels that are unused.

The SN74LVC2G17 is much better choice and smaller footprint.

[cyberosis]

the sync buffer in the DC VGA box isn't the best. I would not recommend it for new designs

switch to the SN74LVC2G17 - it's what I use in my dongles and it's time tested in the OSSC

making that change would also allow you to omit most of the passives

Thanks for the suggestion! Looking at the data sheet it seems all i'll need is the voltage bypass cap and no attenuating resisters or diodes? If true that's way simpler.

[maxtherabbit]

the sync buffer in the DC VGA box isn't the best. I would not recommend it for new designs

switch to the SN74LVC2G17 - it's what I use in my dongles and it's time tested in the OSSC

making that change would also allow you to omit most of the passives

Thanks for the suggestion! Looking at the data sheet it seems all i'll need is the voltage bypass cap and no attenuating resisters or diodes? If true that's way simpler.

yessir

[cyberosis]

Made a new version of the board with the SN74LVC2G17DBVR. The amount of space saved is amazing. The pcb will now sit completely flush with the xbox board, since there's no longer any components underneath!

Spoiler

Thanks again max!

[strygo]

Once installed, you will need to set the Xbox to run in 480p+ and the cable should report as HD AV (component), is that right? And sync will still be present on the sync pin?

[maxtherabbit]

Made a new version of the board with the SN74LVC2G17DBVR. The amount of space saved is amazing. The pcb will now sit completely flush with the xbox board, since there's no longer any components underneath!

Spoiler

Thanks again max!

you're welcome, nice work

[cyberosis]

Once installed, you will need to set the Xbox to run in 480p+ and the cable should report as HD AV (component), is that right? And sync will still be present on the sync pin?

It's a little more complicated then that sadly. For this mod to work you need to use the frozen vga bios. Preferably modified to remove SOG. Syntax released a nice pack but you'll have to tssop flash them on a modded xbox.

https://drive.google.com/file/d/1J3GRF- ... gSgCM/view

tssoping can be a risky process so I use a openxenium modchip which lets you easily load up and switch bios.

https://makemhz.com/collections/xbox/pr ... openxenium

[Syntax]

And I only ever got that hex edit to work on the M8 bios. Works perfectly tho for every game ive tried.

With this board you will be able to run RGBS 15k (No composite for sync or stripper) or RGBS 31k depending on the mode selected via cable choice.

I cannot remember which mode pins need grounding for 31k, ill have to check my gerbers as I remember there's mixed info out there.


MODE 2 and 3 Grounded for 15k RGB
MODE 1 2 3 Grounded for 31k RGB

[cyberosis]

I was thinking how I was gonna easily test all these inputs and decided I needed a breakout board of some kind. So I took the already amazing "Xbox Open Source Video Project" as a base and added my own inputs just for this project. Here's my wip. Excited to experiment with the video filter since its also compatible with RGB.

Spoiler

[Syntax]

I recently issues with the my VGA modded xbox when connecting it to some Extron matrix systems, and wanted to go over the buffer design.

Turns out Xbox only has 1.5v or less for sync and I was just buffering that with the Dreamcast VGA circuit.
Had some spare THS7374 amps and random boards around so put one together in an attempt to double the output and clean it up somewhat.
I thought having the low pass filter off would be best but it created vertical jailbars. So LPF on, 1k and 100n inline for input, nothing on the output (I have diodes and 47pf caps but not really needed and removed them later)
Still have a slight wobble in text with the raw vga into an LCD in 480p, but none of my CRT display this. Raw 720p and up are all rock solid. When combining sync with an Extron unit I get a solid clean picture. Retrotink 5x loves it.

Sitting around 4.5vpp now which is perfect, not sure what else to do...
Im pretty sure the issue is sync polarity is ass about. H is positive polarity.

Spoiler

[truylibra]

Was this buffer board ever completed?