[rescue] Mac IIfx compatible NuBus video card.

Andrew Jones andrew at jones.ec
Mon Oct 26 12:17:18 CDT 2009

On Mon, Oct 26, 2009 at 10:39:42AM -0400, nate at portents.com wrote:
> That's one of two issues.  Either a sync-on-green issue, or a resolution
> issue.
> Until the late-edition Quadra line, Mac video output was sync-on-green,
> and not all LCD screens support it, since composite sync is much more
> common now.

PCs use separate sync: A pin each for H- and V-sync.

Suns use composite sync: one pin for combined H- and V- sync.

Most SGIs and similar RGB hardware use sync on green: one pin for combined H- and V- sync *and* green.

I don't know which one Macintoshes used. I find support for composite and sync-on-green to be unpredictable in LCDs.  The capability basically comes for free in the ADC chips they use, but it's up in the air whether the designers actually planned to use that capability.  

I once had an HP LCD that would work fine with sync-on-green input, but it would display an error message after 10 or so seconds with no signal on the separate sync pins. That is, an unbypassable monitor firmware error superimposed on the perfect-quality image. Definitely a head-scratcher.

Another trick for sync-on-green: clip the H- and V- sync pins out of the VGA cable you use on the other side of your adapter.  Sometimes adapters tie those to funny places, and their removal will make your monitor behave better.

More information about the rescue mailing list