This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
You should keep an eye out for any analog RGB monitor that can horizontally sync down to 15.75 kHz. These include: AppleColor RGB Monitor replacement (model A2M6014, the IIgs monitor) Commodore 1084 models and equivalents like the Magnavox Professional 80 Sony CPD 1302 MultiSynch NEC MultiSync (original), MultiSync II, MultiSync 3D Commodore 1902 (although most of these only do digital RGB) Tandy CM-8 (not sure about model but it was used with the Coco3) Atari SC1224 (used with the ST series) Some TVs For example, I just tried a Sony CPD 1302 multisych monitor on a GS and it works perfectly; and it has a .25 dot pixel! Very crisp picture. I used a standard cable from a Mac II (or you could check with Redmond Cable at http:// www.redcab.com/ ). The NEC MultiSync II is another excellent monitor that will sync down to 15 kHz. (The MultiSync IIA won't work, though.) A cable you can use is shown below:NEC Multisync II Dsub-9 IIgs Dsub-15 1 Red 2 Red 2 Green 5 Green 3 Blue 9 Blue 4 Horizontal Sync 3 Composite Sync 5 Vertical Sync (NC)* 6 Red GND 1 Red GND 7 Green GND 6 Green GND 8 Blue GND 13 Blue GND 9 GND*Note: DO NOT hook up Pin 5 (Vertical Sync); just chop it off. The NEC Multisync II is capable of composite sync on its horizontal sync pin. The Atari SC1224 used with the ST is a ~15 kHz monitor. There were several versions, one by Goldstar, which appeared in one of the two case styles used by Atari over the years and bore the Atari logo. It works fine with the IIgs, provided an adapter cable is created. Redmond Cable can provide it. The Commodore Amiga 1084 monitors and Magnavox equivalents (e.g. Professional 80, 8CM515) will work with the Apple IIgs. You will need a special Dsub-9 to Dsub-15 cable:1084 model with Dsub-9 IIgs Dsub-15 (Analog mode) 1 GND 13 GND (Blue) 3 Red 2 Red 4 Green 5 Green 5 Blue 9 Blue 7 Composite Sync 3 Composite Sync1084 model with DIN-6 IIgs Dsub-15 Analog Connector 1 Green 5 Green 2 Horizontal Sync 3 Composite Sync 3 GND 1, 6, 13 GND's 4 Red 2 Red 5 Blue 9 Blue 6 Vertical Sync (NC)**Note: DO NOT hook up Pin 6 (Vertical Sync). Just chop it off. By: Michael Hickey, G. Utter, Jeff Blakeney, Steve Jensen, Blake Patterson, Scott Alfter, Louis Schulman, James Keim, Faisal Karmali, John Fenske ---------------------------- By: Ruud Here's a different approach: my Powermac 7500 is right next to my IIgs; so, I plugged its composite video output into the video-in port of the 7500, started Apple Video Player, and lo and behold, it is possible to use your 1024*768 multires RGB Powermac monitor as a IIgs monitor, in a pinch. Color is all washed up, but mono is OK. By: Stefan Voss, Joakim Ögren