Actually, IBM did not supply a TTL-monitor for CGA until 1983 (the 5153).
So in the early days, using the composite output (which is standard on IBM CGA cards) would be the easiest way to get CGA output to a screen in 1981.
I stand corrected. :) Did those cards have an RCA connector? I came across a 64K full-width EGA card that had two of those, but never figured out what sort of output they produced.
They did, and it worked! The RCA connectors on the IBM EGA card aren't connected to anything - they only work if an add-on daughterboard is installed (I don't know if that daughterboard was ever actually made).
Yes, CGA has an RCA connector on the back for NTSC composite. This is what we used for the demo.
There is also a header on the card itself, to which an RF modulator can be connected, for a regular TV antenna input (this was an option, probably because of regulations. An RF modulator generates a lot of interference, so fitting one stock would require expensive shielding. Fitting it 'aftermarket' circumvents that).
The IBM 5155 portable PC abuses this header to connect the composite signal to an internal monochrome CGA monitor.
