The O2 is known to output sync on green signal (hsync and vsync embedded in the green channel). For some reason it also outputs separate hsync and vsync signals, which can cause various issues.
If you get something like this (permanent or random when starting up the machine):
You are in one of these 3 cases:
1. Your monitor supports sync on green, but the image always has a tint of green.
This happens because, even though the monitor supports sync on green, it sees the separate hsync and vsync signals provided by the O2, and decides to use those instead of sync on green.
Because of this, it does not correctly interpret the green channel, causing a green tinted screen.
Fix for this is simple, take the VGA cable, and remove the pins that carry HSYNC and VSYNC (13 and 14). Only need to remove them on one side of the cable.
My Dell SE2416H always displayed a green tinted image, and according to the manual it does not support sync on green. However, this is how it looks after removing the hsync and vsync pins:
So it is worth a shot even if your monitor does not officially support Sync On Green (at the cost of a VGA cable).
2. Your monitor supports sync on green, but the image sometimes looks OK, sometimes has a green tint.
Same as the above, sometimes the monitor decides to go on the separate syncs route, and the green doesn't get decoded properly.
Fix is the same, remove HSYNC and VSYNC pins (13 and 14) from the VGA cable.
3. Your monitor does not support sync on green.
In this case, the O2 can be persuaded to output a standard VGA signal, without sync signals embedded in the green channel.
This is a hardware mod, it might be possible to convince the GBE/EVIL to stop outputting the composite sync signal, but I have no clue how to do that at the moment.
The O2 uses an ADV7120KS140 DAC. It is located near the VGA connector, here:
There is a datasheet available for the ADV7120, however, it does not at all match the QFP variant found on the O2. It might be a special P/N made for SGI.
The QFP variant on the O2 does match however the PLCC version in the datasheet, like this:
There are a couple of ways to disable sync on green:
3A (easiest approach and can be easily undone).
Lift up the ISYNC pin (pin 31 of the QFP package).
3B (somewhat reversible but doesn't require a soldering iron).
ISYNC pin is connected directly to the GREEN channel output of the DAC. If you look closely at the photo, there is a trace that connects pin 32 and 31). This trace needs to be cut.
3C. Lift up the !SYNC pin (pin 10) and connect it to ground (grounding might not actually be needed,it worked fine on my end with it floating, but the datasheet does not mention any internal biasing for this pin. Your mileage may vary).
By doing 3A or 3B or 3C, the O2 will output standard VGA signal, instead of sync on green. No VGA cable modifications are needed (in fact, it won't work if the hsync and vsync pins are removed).
Here is how the O2 looks with 3A/3B/3C, a standard VGA cable, and a monitor that before displayed a green tint:
If you get something like this (permanent or random when starting up the machine):
You are in one of these 3 cases:
1. Your monitor supports sync on green, but the image always has a tint of green.
This happens because, even though the monitor supports sync on green, it sees the separate hsync and vsync signals provided by the O2, and decides to use those instead of sync on green.
Because of this, it does not correctly interpret the green channel, causing a green tinted screen.
Fix for this is simple, take the VGA cable, and remove the pins that carry HSYNC and VSYNC (13 and 14). Only need to remove them on one side of the cable.
My Dell SE2416H always displayed a green tinted image, and according to the manual it does not support sync on green. However, this is how it looks after removing the hsync and vsync pins:
So it is worth a shot even if your monitor does not officially support Sync On Green (at the cost of a VGA cable).
2. Your monitor supports sync on green, but the image sometimes looks OK, sometimes has a green tint.
Same as the above, sometimes the monitor decides to go on the separate syncs route, and the green doesn't get decoded properly.
Fix is the same, remove HSYNC and VSYNC pins (13 and 14) from the VGA cable.
3. Your monitor does not support sync on green.
In this case, the O2 can be persuaded to output a standard VGA signal, without sync signals embedded in the green channel.
This is a hardware mod, it might be possible to convince the GBE/EVIL to stop outputting the composite sync signal, but I have no clue how to do that at the moment.
The O2 uses an ADV7120KS140 DAC. It is located near the VGA connector, here:
There is a datasheet available for the ADV7120, however, it does not at all match the QFP variant found on the O2. It might be a special P/N made for SGI.
The QFP variant on the O2 does match however the PLCC version in the datasheet, like this:
There are a couple of ways to disable sync on green:
3A (easiest approach and can be easily undone).
Lift up the ISYNC pin (pin 31 of the QFP package).
3B (somewhat reversible but doesn't require a soldering iron).
ISYNC pin is connected directly to the GREEN channel output of the DAC. If you look closely at the photo, there is a trace that connects pin 32 and 31). This trace needs to be cut.
3C. Lift up the !SYNC pin (pin 10) and connect it to ground (grounding might not actually be needed,it worked fine on my end with it floating, but the datasheet does not mention any internal biasing for this pin. Your mileage may vary).
By doing 3A or 3B or 3C, the O2 will output standard VGA signal, instead of sync on green. No VGA cable modifications are needed (in fact, it won't work if the hsync and vsync pins are removed).
Here is how the O2 looks with 3A/3B/3C, a standard VGA cable, and a monitor that before displayed a green tint:
