Connecting a standard PC-Monitor to an SGI causes usually no problems as long as it supports sync-on-green, sometimes also referred to as composite-sync signal. All usual high end monitors and the better flat screens support sync-on-green, a list of compatible monitors is available at the Linux for Playstation 2 website.
If you are lucky and have an SGI 13W3-to-VGA cable at hands, your monitor will most likely work like a charm.
But what happens if you only have on of those generic SUN 13W3-to-VGA adaptors at hand? They would work if you plugged in a SUN box, but fail when attaching an SGI. The reason for this is simple: SUN and SGI have different a different pinout on the 13W3 port, which prevent interchangeing the adaptors.
So if the only thing you have is a SUN 13W3-to-VGA adaptor and you really need to connect your SGI, the only way is to break some of the pins from the 13W3 side. Although my web searches revealed that actually only one pin had to the broken off, I never found which one it was. It’s like one of those things everyone seems to know about so nobody would actually tell you…
So after digging around some hundred entries in the newsgroups I found a consensus between two variants which apparently should lead to success. One stated that all small pins should be broken off, leaving only the three RGB (the coaxial) pins. The other statement was to break off all non-GND pins.
So I decided to go with the second approach first, breaking off only the non-GND pins. After checking with the SGI techpubs library I got on hold of this pinout scheme:
Beware that this image shows the pinout of the connector on the box, so when looking at the adaptor it’s actually reversed angle. I used a screw driver to remove the non-GND pins: 1, 2, 3, 4, 6 and 7.
Finally the SGI PROM monitor appeared on screen after attaching the modified adaptor.
Somebody was actually very happy when this worked out 🙂
P.S. The modified adaptor would not work on a SUN aftwards. As stated earlier only one pin must really be broken off which possible allows dual-use of the adaptor on both SGI and SUN boxes. As to my knowledge two of the pins are looped to allow a SUN to detect a monitor. Since I didn’t bother in going through the SUN tech sheets this is unverified information. If somebody out there knows it please drop me note.