I wanted to use a SiliconGraphics GDM-17E11 monitor on my Sun Ultra 10. My Syn Ultra 10 has a vga output, and the monitor has a 13W3 input, which requires Sync-On-Green. That's why I needed to find a way to convert VGA to RGB SyncOnGreen.
Usually, there are 5 important signals coming out from a VGA card. The colors (Red, Green,Blue) and Sync
(H-Sync and V-Sync). The voltage on the color signals is proportional at the brightness of the color at
a given moment. The Sync signals are generally kept high, with short low pulses when retrace must occur.
Composite Sync (C-SYNC):
Some monitors support or need Composite Sync (C-Sync). C-Sync is both H-Sync and V-Sync pulses combined
together with a simple circuit(dont try connecting the 2 wires together). The monitor can differentiate
between H-Sync and V-Sync by looking at the pulse width. The V-Sync pulse is longer. Old sun monitors required
C-Sync, and I guess that's why C-Sync comes out of my Ultra 10 instead of V/HSync.
For some info about how to convert H/V-Sync to C-Sync, consult this page:
VGA to RGB + composite sync -converter
Some monitors requires the C-Sync pulses to be combined with the Green color signal. That's the case
with my GDM-17E11. It can be done with a very simple circuit, as demonstrated on this page:
Circuit for converting spearate sync signals to SYNC-in-GREEN.
Most schematics includes a part to combine V and H sync, but sometimes it is not necessary. Some video
cards can be programmed to output C-Sync on the H-Sync pin (+csync). I case you are wondering, the sync
pulse does not interferate with the green signal because the pulses always occur at the end of a line
or at the bottom of the screen.
As you can see on the picture above, it is easy to recognize which pulse is H-Sync and V-Sync. When I took
this sample, my screen was totally black, but there was a small taskbar in the bottom. We can see how
the task bar effects the voltage of the green signal. We can also confirm that the sync pulses occur
at the end of a line by looking at the vertical line I drew.