Q: Can I attach two monitors to my Rainbow?
Yes, you actually can. When the graphics option is present, the Rainbow is capable of operating with two monitors. According to the Rainbow Color/Graphics Option Programmer's Reference, the Rainbow's hardware is capable of controlling both a color monitor and a mono monitor simultaneously. The Rainbow's pinouts also seem to suggest this. If two monitors are connected, the Rainbow routes all graphics to the color monitor, while sending all text to the monochrome monitor. Sadly, DEC never made a cable to take advantage of this feature, but one can be easily built.
The software must be aware of this configuration. Normally, text is output on the monochrome pin. This leaves the Rainbow's green pin unused on a color monitor. In fact, the standard DEC cable that connects the VR241 Color Monitor to the Rainbow utilizes the monochrome pin for the green graphics. When switched to graphics mode, the green signal is still not used; the green signal for the color monitor is taken from the monochrome pin. However, if the software is configured properly the green signal can be used by the color monitor when switched to graphics mode, leaving text on the monochrome signal. DEC did not support this in many packages due to the difficulty of attempting to support three screen configurations.
Some CAD packages, GSX-86, and a few other programs can take advantage of two monitors without any modifications. Other software can be modified to work properly in a dual-monitor configuration with some slight modifications of the binary code.