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 Post subject: Original Xbox VGA Sync buffer pcb
PostPosted: Thu Feb 18, 2021 12:12 pm 


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Joined: 28 Feb 2019
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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.

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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


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

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

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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Tue Feb 23, 2021 11:38 am 


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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
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PCB
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test fit
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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Tue Feb 23, 2021 4:47 pm 


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Joined: 09 Aug 2017
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Location: Australia
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


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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Wed Feb 24, 2021 3:00 am 


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Joined: 28 Feb 2019
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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.
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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.


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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Wed Feb 24, 2021 7:23 am 


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Location: Australia
Nah, do it properly ;)

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


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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Wed Feb 24, 2021 12:06 pm 


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Joined: 28 Feb 2019
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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
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PCB
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Test fit
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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Thu Feb 25, 2021 3:38 am 


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Joined: 05 Mar 2018
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did you just pick two unused pins on the multi out for HV sync or is there some precedent for using those?


Last edited by maxtherabbit on Sat Feb 27, 2021 2:20 pm, edited 1 time in total.

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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Thu Feb 25, 2021 4:34 am 


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According to the GameSX pinout, these are the pins usually used for V and H sync in vga mode.
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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.
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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Sat Feb 27, 2021 11:53 am 


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

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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Sun Feb 28, 2021 11:59 am 


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


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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Sun Feb 28, 2021 2:39 pm 


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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


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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Mon Mar 01, 2021 1:01 am 


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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.


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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Mon Mar 01, 2021 4:20 am 


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maxtherabbit wrote:
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.


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 Post subject: Re: Original Xbox VGA Sync buffer pcb
PostPosted: Mon Mar 01, 2021 2:32 pm 


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cyberosis wrote:
maxtherabbit wrote:
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


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