The COMX 80 Column card adds the possibility to use a text mode of 80x24. The MC6845 is used as video chip.
The 80 Column Card includes a 2K ROM mapped to location @C000-@C7FF this ROM includes firmware to extend the COMX BASIC with support for the 80x24 text mode. Next to this an additional 2K Video RAM is included and mapped to location @D000-@D7FF. The MC6845 can be addressed via memory locations: @D800 for the CRTC address register and @D801 for the CRTC data register.
When the 80 column card is connected to the COMX the BASIC ROM still starts up in regular mode. This makes it difficult to start the COMX if no monitor/TV is connected to the regular output. To start the 80 column mode either a CALL (@C010) or when using the expansion box a CARD Sx (x = 80 column rom slot) should be used. After this command BASIC will change output to the MC6845 chip on the 80 column card.
There is not much software available supporting the 80 column card, to my knowledge the COMX Star text editor was released with 80 column support however I don’t have a copy of that specific version.