This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
First, I should correct a misconception: the video port on the back of the IIc+ is not an RGB port. It is a video expansion port, which provides all of the internal video generation signals used by the IIc/IIc+ which can be used to generate an alternative video output signal. The actual functions of the IIc video port are as follows:1 TEXT Indicates text mode is active (spcl fn in DHR mode) 2 14M 14 MHz clock sigal 3 SYNC Horizontal and vertical sync 4 SEGB Vertical counter signal from IOU, or lo-res indication 5 1VSOUND Sound output (one volt peak-to-peak) 6 LDPS Video shift load enable 7 WNDW Active area display blanking 8 +12V +12 volts DC 9 PRAS RAM row address strobe 10 GR Graphics mode enable 11 SEROUT Serialized character generator output 12 NTSC Composite NTSC video output 13 GND Ground reference 14 VIDD7 Bit 7 of video shift latch (hires mode col shifting) 15 CREF Colour reference timing signalThese come from the IIc Technical Reference, both first and second editions. You cannot connect a monitor directly to the IIc video port (with the possible exception of the the LCD display, or an NTSC monitor). To produce RGB output (or anything else) from this port, you need an external adapter box. This adapter is not simple: it has to decode the colour information from the NTSC video signal (or generate it by detecting the graphics mode and monitoring bit patterns), generate appropriate sync signals, etc. I believe there is (or was) an RGB output adapter for the IIc, which should also work on the IIc+. By: David Empson