OSSC Pro and X68000

[darkwitchclaire]

I have an Acer monitor that works great natively connected to my X68000 via VGA, but when I tried it with my OSSC Pro it refused to display the signal. I tried forcing 2x and checking out some of the scaler options but no matter what I did I got no image. Are there some tweaks that would help with this? It shows a 568p signal on the OSSC Pro’s display so I’m not sure what it’s outputting that would cause this.

[BuckoA51]

Just to clarify, it’s the X68000 you’re trying to connect to the OSSC, and then the OSSC to… which display?

Are you at least seeing the test card pattern on your target display?

[marqs]

So far X68k is the only known machine with which the ADC has had trouble locking the sync reliably. I’ve only been able to shortly look into one but could not see any irregularities in its sync signal. This particular case was solved by a sync combiner so my suspicion lies in the sync level circuit recommended by the manufacturer. 568-p in itself is correct for X68k.

[darkwitchclaire]

I’m trying it through the OSSC for capture purposes and being able to play nonstandard resolutions at the proper aspect ratio, since which it handles 15khz fine there’s no way to adjust the aspect ratio. I’ve also tried it with my Dreamcast over VGA and it works great.

[Meramillion]

This is a compatibility report for X68000 and OSSC Pro.
I have two pieces of hardware, an X68000 XVI and an XVI Compact, both of which display fine in OSSC 1.7 with a signal from the VGA.
The input from the X68000 to the OSSC is via a Dsub15 connector conversion.
However, OSSC Pro will display the X68000 XVI Compact but not the X68000 XVI.
If there is a difference between the XVI and XVI Compact hardware, the Compact has a VGA mode, but since it is currently used in normal mode, there seems to be no difference.
Do you know why only the XVI is not displayed?

[marqs]

From what information I could find, there are minor differences on the video output circuit between the models. It looks like standard/XVI models use 74ALS05/74LS06 IC to drive sync signals while Compact uses 74ALS04. The former are open-collector type while latter not which indicates that pull-up circuit consisting of resistors and diode would be omitted on Compact. Not sure why this has such an impact on Pro, but several things are pointing to a design flaw in the video ADC chip which makes it very sensitive to H/V sync signal quality despite internal schmitt trigger and glitch filter.

Simple tweaks like removing c41 and c42 from Pro board are under investigation, but it might be that a sync buffer like SN74LVC2G17 needs to be added on a future board revision to completely fix the issue. Easiest non-DIY solution to get X68000 working with Pro is currently using a sync combiner like Kenzei / UMSA to connect the signal to SCART.

[Meramillion]

Thank you, marqs.
I had no idea there was such a difference.
If I eliminate c41 and c42 from the OSSC Pro board, will they show up correctly without using the sync combiner?
I will try to do a DIY.

[marqs]

If I eliminate c41 and c42 from the OSSC Pro board, will they show up correctly without using the sync combiner?
I will try to do a DIY.

I should have answer to that in maybe a few weeks. At least it has not had negative impact on any TTL sync source I’ve tested.

[Meramillion]

Thank you, marqs.
Understood.
I’ll use the sync combiner to connect until I get a solution.

[darkwitchclaire]

Would the VGA2SCART also work as a sync combiner? It’s cheaper than the other options you mentioned.

[marqs]

It could not find a schematic for it, but it sounds very similar to the other options so it should do the job.

[marqs]

If I eliminate c41 and c42 from the OSSC Pro board, will they show up correctly without using the sync combiner?
I will try to do a DIY.
I should have answer to that in maybe a few weeks. At least it has not had negative impact on any TTL sync source I’ve tested.

Removing these did not help according to one tester report. I have another HW fix available which will get tested with X68k in couple weeks.

[darkwitchclaire]

Got a VGA2SCART and confirms it works.

[Meramillion]

Hello marqs.

Removing these did not help according to one tester report. I have another HW fix available which will get tested with X68k in couple weeks.

Have you made any progress on this matter?
I am looking forward to seeing the X68000 XVI on OSSC PRO’s VGA.

[marqs]

As mentioned in the NAC Splitfire thread, the fix (which involves soldering SN74LVC2G17 on the board) has been tested also on X68k and reported working. I will create some instructions next week for those brave enough to try it out. It’s not a hard mod, but considering how many people have had issues desoldering 0603 parts while doing OSSC Classic mod for v1.xx firmware, I can recommend this one for somewhat experienced modders only.

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