Reply 320 of 457, by jal
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wrote:Well, it was too late to change the CGA specifications, but the exclusive display monitor the device was supposed to use, the 5153 Color Graphics Display, could be changed.
The CGA specification had little to do with it, as, as already pointed out in this forum, the CGA sends a 4 bit digital signal to the monitor. The monitor then does the D/A conversion, and produces a certain color.
wrote:When IBM designed the EGA, it did so with the intent to make it fully-backwards compatible with the CGA and CGA monitors.
I'm not sure they did. I know of no EGA card with a CGA connector, did the original EGA have this? EGA has no I-line, but emulates this through its 6 color lines (RrGgBb). Theoratically, it's possible to connect a CGA monitor to an EGA card, but I doubt IBM intended for this to be commonplace.
wrote:This meant that in 200-line modes, the EGA would have to be restricted to the 16-color CGA palette even though EGA could generate 64 colors. If IBM allowed the programmer to manipulate the palette registers in the 200-line modes, it would not show the proper color on a CGA monitor.
Yeah, and you can easily 'blow up' your CGA monitor by switching to 350 line mode. I really doubt this is why IBM decided not to support 64 colors in 320x200 mode. I could be wrong though, of course.
wrote:(I would guess that for text modes you would have to set a dipswitch so that the EGA uses a 200-line text mode instead of the usual 350-line text mode, otherwise you would need a second monitor for the text mode or risk damaging your CGA monitor.)
I know of no such dipswitch on the original EGA card. Are you sure you are not making this up as you go?
JAL