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Sep 29, 2022
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So I've been wiring up my NAC to be ultimately flexible, able to run anything, and part of that 'anything' includes my Naomi board. After whipping up a power cable for the Sega I/O board, I plugged it all in and it worked perfectly, but for two problems: the VGA cable rapidly turned into a heating element, until I clipped pin 9. Yikes.

And then the the sync. The Nanao MS9 chokes on the sync, being stable horizontally but not vertically. So I assume that, in 15kHz mode, the Naomi is still spitting out separated sync, and that I need to combine it.

Can anyone confirm this, and has anyone got a solution?


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If you are using a sega IO with the Naomi in 15khz mode it should just work you dont need anything for sync cleaning and such. Clipping pin 9 is a very good idea because with some monitors that pin is used for something else and it will burn a trace on the filter board. (This coming from hands on experiance on all fronts from me telling you this)
Maybe your vertical hold/sync is ever so slightly off on the chassis and the naomi setup is just enough to make it roll vertically???

Otherwise idk what to tell you unless you have another Naomi or similar to start ruling parts out.
If you are using a sega IO with the Naomi in 15khz mode it should just work
To be clear, this is the JVS I/O input board, for the panel inputs into the Naomi via the USB-like cable. I'm not using a Sega JAMMA adaptor.
Can you post a picture of what exactly it is you're working with? I suspect there's a disconnect somewhere with the terminology being used.

The behavior you're describing does indicate it's a H/V sync issue - Naomi does not output combined sync. This is usually solved by feeding video into I/O boards (Capcom, Sega) that output combined sync over the JAMMA edge.
Sure thing. Here's the setup, looking not entirely unlike a cabinet that's puked its guts up all over the floor. ^__^

But yeah it seems like I need to make a sync combiner. There's absolutely no point buying a $300 JVS/JAMMA board when $5 in parts will do the trick.


are you running the output of the naomis VGA directly into the RGB input of the monitor with a wiring adapter??

Thats not possible unless you have a tri sync chassis that allows for this and the photo looks like an astro. The video level over VGA is also different compared to normal RGB that the monitor is expecting.
Standard Sega JVS to JAMMA adapter boards can be had for like 45 to 75 US dollars and will handle all the video conversion to RGB levels over JAMMA for you.

That said you will need some kind of IC like a sync combiner or something to make that work and you should correct your video level as well. VGA uses like 0.7V and RGB is 3.0V if I recall off the top of my head. I guess you could compensate by boosting your contrast and stuff but its not ideal imo.

Look into converting VGA into RGB for sync and video level voltage / impedence Level
Have to agree with Geekman that buying a $60 Sega I/O is probably less work (and cleaner from a I/O wiring perspective), but rolling your own sync combiner is certainly an option. Here's one on ebay right now for $50 USD plus ship: https://www.ebay.com/itm/284404900132 - There should be a few sellers in AU that have them in stock, but unsure of cost. You can also get them from Japan for next to nothing.

Also +1 to Geekman's comments regarding video levels, I believe JAMMA RGB is like 4V peak to peak.
I mean I hope im not coming off as mr. "well thats not kosher" lol. I commend the effort going into this truly.

That said just from the stand point of doing all this I feel like for the effort and time I personally would just throw out 40~ USD for a bog standard Sega JVS to Jamma adapter board. I used it on my Astro its honestly just plug and play and also gives you some options for adjusting how button 4 and 5 are routed incase you were wanting to run a kick harness off the IO into your cabs Kick/Aux input connector on the main loom. The IO is also powered off the main Jamma loom so one less thing to make power cables for. The only thing it wont do is stereo amplification and providing JVS 3.3 power voltages to run the Naomi. You already have those bases covered though
40~ USD for a bog standard Sega JVS to Jamma adapter board.
That's not unattractive. I thought they were all Capcom-level pricing, so I never considered the option. I was almost to the point of attempting to shove the JVS board and all the associated sockets into the Naomi itself, to make it - if not consolized then at least less messy.

My thinking was, as you can see in my cables post, that I was going to make this hella modular and flexible af. But I'd been doing all my testing on a Naomi Universal, which has - it seems - a normal VGA monitor. No need for all the sync messing around and levels manipulation. I did not expect to run into this problem on the NAC, in my mind it was all '15kHz means no upscan means less hassle' but oops, live and learn.
Hello Everyone,

Thank you for this post.

I have an Astro City with a VGA only monitor.

I use a Capcom IO to connect my Naomi to the monitor and Jamma.

This is a wipi setup with OpenJVS. When I disconnect the USB B from the Capcom IO to connect it to the OpenJVS hat, the monitor loses sync. From this post I understand that an IO needs to be there to sync the image.

In this case, how can the image be synced if I can only connect the Naomi to either the Capcom IO or the OpenJVS?

What would be the solution for this setup?


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