Silicon Graphics / IRIX 13W3 and Sync on Green (SoG) solutions

fraserN64

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I picked up a 13W3 to VGA cable with the DIP switches, and the image in OP is helpful but I'd like to clarify what to do with the switches that show ON and OFF? Does it not matter? In which case I've only got #6 switch set to ON and the other 11 set to OFF.
Can anyone take a photo of their working DIP settings for SGI SOG? Thanks.
 

Elf

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Removing the sync pins on the VGA side of a BNC to VGA adapter? I would think the easiest solution on a set of BNC to BNC cables, or a BNC to VGA adapter that has separate BNC cables for H/V sync would just be not to connect the H/V sync to anything if you don't want it.

If it's a BNC to VGA adapter that connects the Green channel to H/V sync internally, then removing the pins might help for some monitors?
 

indigofan

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Jun 8, 2020
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Removing the sync pins on the VGA side of a BNC to VGA adapter? I would think the easiest solution on a set of BNC to BNC cables, or a BNC to VGA adapter that has separate BNC cables for H/V sync would just be not to connect the H/V sync to anything if you don't want it.

If it's a BNC to VGA adapter that connects the Green channel to H/V sync internally, then removing the pins might help for some monitors?
I acquired a SUN BNC to 13w3 cable and want to get it to work on an SGI. The monitor has BNC connectors and is a SGI Monitor. I believe from reading that I can remove Pin 7 and it should work. The LCD panel that I have does not support SOG. So just trying to figure it out before I start popping off pins on the cable.
 
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Elf

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Oh, I see! BNC to 13W3, but Sun configuration. I apologize, I am not sure what would work in that situation, or what would be different between Sun and SGI cables.
 

indigofan

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Oh, I see! BNC to 13W3, but Sun configuration. I apologize, I am not sure what would work in that situation, or what would be different between Sun and SGI cables.
It requires some surgery..

 
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Jamieson

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My Indy (8 bit graphics) is paired up with an older NEC LCD display that does not support sync on green. To get around this I wired up the H and V sync pins from the 13W3 connector to the VGA connector. Now the monitor syncs up, but it has a green tint. The reason for this is that the sync signal is added to the green video signal by default, and this shifts the entire video signal up from ground by a few hundred mV. That confuses my monitor into thinking that green video is always on at ~50% level, resulting in the green tint. The sync signal is added to green by the RAMDAC chip, in this case BT445. There is a pin on that chip called SYNC# and according to the datasheet, tying that pin to GND would disable the sync and restore the green video level back to normal. (One would have to lift the pin on the BT445 and carefully solder it to GND to make this work.) Now if there is a way to do this in software or with some ENV command that would be cool, but at some point I'll look into doing this hardware mod to get rid of the sync on green.
 
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Jamieson

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Here is the relevant part of the Brooktree BT445 RAMDAC datasheet. When SOG is enabled the green video signal is shifted up ~300mV relative to red and blue. Interestingly this RAMDAC device has a few internal registers and command register 1 bit 7 controls the SOG feature. So maybe a hardware mod is not necessary... that is if SGI provides a way to access these internal RAMDAC registers. Otherwise the hardware mod is to lift pin 147 (SYNC#) on the BT445 and tie it to GND to eliminate SOG.
 

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Elf

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That also lines up with my understanding of why some monitors have a green tint when receiving Sync on Green. Sometimes they behave better if they H/V sync pin on the cable is removed and they are not receiving the sync in both ways. Or, one can use a more complex DC clamp circuit to remove the sync pulses as well.

Regarding software access, have you tried the setmon command above (on the first page of the thread)? It probably would not help before IRIX boots but it seems to give you the ability to change the sync output from within the OS.
 

ghost180sx

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Here is the relevant part of the Brooktree BT445 RAMDAC datasheet. When SOG is enabled the green video signal is shifted up ~300mV relative to red and blue. Interestingly this RAMDAC device has a few internal registers and command register 1 bit 7 controls the SOG feature. So maybe a hardware mod is not necessary... that is if SGI provides a way to access these internal RAMDAC registers. Otherwise the hardware mod is to lift pin 147 (SYNC#) on the BT445 and tie it to GND to eliminate SOG.
That should be easy to solve. You can tell the Indy to output H+V sync and disable SoG. See my post #4 in this thread!
 
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lohmos

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Oct 19, 2022
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There was a prom variable crt_option=1 that worked on O2 for ugly greenish screens but on Octane this variable is not known by prom
 
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