SNES sync issues after mod

[blackice85]

Got it, thanks. I think making the modification to Voultar’s board is probably the safest and easiest for me (ie least likely to screw it up).

[marqs]

CSYNC on SNES multiAV (where available) comes from an emitter follower circuit that outputs ~2.5 volts through 100ohm series resistance. Even if that is directly connected to 75ohm termination, it drops to safe ~1.1V level. However, if Voultar’s board uses THS7374 to drive csync at 5v without series resistance (in TTL mode), it can cause issues. As said above, easiest thing to try is opening the TTL jumper on the board.

[BuckoA51]

So would you say when dealing with csync from a standard SNES that the resistor on the SCART cable/csync output is unnecessary then? Or perhaps strictly speaking unnecessary but recommended?

[blackice85]

Looks like that did it, sync is rock solid now. Not always the exact frequency each time I turn it on, but it doesn’t move anymore. It’s a bit lower than when it syncing intermittently before too, and about where it was pre-mod if memory serves. I guess before sometimes it just happened to be at the right levels to sync and sometimes not. Also all output modes are working again, not just x3 and x4.

Thanks for the help everyone.

[eric90000]

Great to hear it! So to confirm, there’s no components on the sync line in your cable, and you left the TTL strap OPEN on Voultars board (75ohm)?

I’ll have your same setup pretty soon!

[blackice85]

Yep, unless there’s a hidden resistor elsewhere in the cable, there’s nothing on the sync line. So I guess I just had an older cable, and didn’t know enough to specify to Voultar.

Good luck with your soldering though, it’s definitely precision work. I’m glad I had a professional do it in my case, that kind of detail is well outside my skill level. You should be pleased with the end result however, that RGB board is great.

[Voultar]

Woo! I’m glad you got this sorted.

The reason I provide the option of handling C-Sync attenuation on the board is because not all SNES “C-Sync” cables are built the same. A lot of these cables from a few years ago are strictly pass-through with zero attenuation network.

The C-Sync off of the SNES board is driven directly off of the ASIC, it’s intended for high-Z terminations. It’s strictly intended for TTL devices, or with cables that have a current limiting resistor. The C-Sync output off of the 1-CHIP ASIC is an open-collector. So if you plug a SNES into a 75term device in TTL mode, it’s going to sink current and pull things down. Short the TTL jumper for high-Z load devices or if your cable has a series resistor to mitigate current-sink. I’ve found that the buffered C-Sync state off of the S-RGB doesn’t totally agree with a few TTL dependent devices. After source termination, the sync drops to roughly 2.5vPP.

(Damn, I need to calibrate the old girl!)

Since you had no series resistor in your SCART cable, the C-Sync signal is sitting pretty high, displacing the slice level of the trigger range on the OSSC. With the TTL jumper “open”, C-Sync will live around 500mVPP. I don’t know where the OSSC slices, but the bigger the signal, the higher it lives. So your intermittent sync issue makes perfect sense here.

And of course, Video will live right around 700mVPP @ 75ohm impedance, source terminated. 🙂

SCART cable standards are non-existent as they’re entirely balkanized. I decided to add this little feature over a year ago to accommodate all of the various flavors.

All systems that are sent to me leave outputting logic-level, high-Z c-sync. Everyone should just sync on luma (to avoid chroma coupling) anyways.

[blackice85]

Ah that makes sense then. Like I said, had I known better I’d have said something to you when placing the order, but the TTL vs 75ohm thing never came up for me before, so I didn’t think I needed anything special. Glad it was a simple mistake on my part.

[marqs]

@blackice85: sync frequencies displayed on character LCD may be slightly off time by time, no need to worry about that.

@BuckoA51: the resistor may be omitted only if sync is wired from unmodified multiAV output (assuming all revisions have similar driving circuit), not if it’s tapped elsewhere from the mainboard as noted in Voultar’s post.

@Voultar: did I understand right that your board does not use THS7374 for buffering c-sync, but bypasses sync output of S-RGB chip to multiAV directly / thru series resistor depending on TTL jumper?

[Voultar]

@voultar: did I understand right that your board does not use THS7374 for buffering c-sync, but bypasses sync output of S-RGB chip to multiAV directly / thru series resistor depending on TTL jumper?

Sorry, I misspoke on that. 1CHIP systems still employ that emitter follower circuit to drive C-Sync from the ASIC. With the 1CHIP-03, those components are omitted from assembly and aren’t connected.

[eric90000]

Has it been confirmed if a SNES RGB cable with 75ohm attenuation works with the TTL setting on Voultars board?
Obviously the 75ohm setting on voultars board works with a sync passthrough cable as evident by this thread.
I’m just trying to decide which RetroGamingCables sync option to go for in my new cable, which will be used with Voultars bypass board:

1. CSYNC attenuation for 75Ω operation

Or

2. TTL sync for RGB modified consoles which have internal attenuation set already

[Voultar]

That’s precisely what the board is designed for. If you have a cable that attenuates TTL C-Sync, short the TTL jumper on the board. and you’ll be dipping into 400mV-600mV, which is ideal.

[eric90000]

Ah perfect, makes sense! I’m assuming the same rules apply to your N64 board? Are 75ohm attenuated SNES cables appropriate for that also?

[Voultar]

That’s correct.

[eric90000]

Unfortunately I’m having issues with my SNES mini w/ Voultars RGB bypass board into an OSSC. Cable is 75ohm Packapunch SNES cable from RetroGamingCables, so has resistor on sync line. Shorted TTL on Voultars board and getting sync dropouts on OSSC. Tried opening TTL jumper and still same results.

It’s worth nothing that I have installed an ‘Ultimate SuperCIC’ board by Bad_Ad84 over on the Assembler forums. This board has new timing crystals for both PAL and NTSC. Could that be affecting things?

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