EGA is not fully compatible with CGA. For starters, it does not support the CGA palettes. Also, some CRT-registers are different, so certain tweaks (eg 80x50/80x100 textmode) do not work the same on EGA.
As for composite, I would assume not. Firstly, since it doesn't support the CGA palettes, you won't get proper composite colours there, even if the composite circuitry is the same otherwise. Secondly, given that even most CGA clones (ATi, Paradise, PCjr/Tandy) don't have the same composite colours as real CGA, I don't think EGA cards would match the composite of real CGA cards either.

hmmmm I knew things wouldn't be that easy 🤣
what about the monitor question? I assume a CGA one won't work with an EGA card, I see in wikipedia that the video-out pinouts are different... maybe an adapter of some sorts exists?

keropi wrote:

what about the monitor question? I assume a CGA one won't work with an EGA card, I see in wikipedia that the video-out pinouts are different... maybe an adapter of some sorts exists?

CGA monitors do work actually. EGA has a compatibility mode where it uses only 200-line modes, not the new 350-line high-res modes.
This is also why 320x200 mode can't use the extended 64-colour palette, but only the standard 16-colours from CGA.

so it's as I feared, you need the right combo of same card/monitor in order to be trouble free... thanks for the info!

The best combination would be an OEM IBM CGA PCB and 5153 CRT. 🤣

keropi wrote:

so it's as I feared, you need the right combo of same card/monitor in order to be trouble free... thanks for the info!

Well, not entirely. The EGA monitor is fully backward compatible with CGA. So you'd only need one RGB TTL monitor, just two cards.
Of course, for CGA you'd want composite, so you'd still need another monitor or TV for that 😀

Finding an RGB TTL monitor can be quite hard these days. I've found that a VGA card that can work in an 8bit slot helps a lot. Furthermore and the most important of all is that some VGA chipsets could switch to CGA compatibility mode. Some are better than others, i've tried a few with decent results:

Trident 8900C
Trident 9000i
Tseng ET4000
Western Digital WD90C30
OAK OTI-067

And there are others. When i was on vacation i had the chance to crystal mod my 286 and run it at 2MHz, which was just marginally faster than an IBM XT. So i fired up probably one of the games that require a true IBM CGA to function like it was intended: Round 42.

The game worked on a ET4000 in CGA despite looking a tad wrong, but not enough to make the game unplayable or look bad. It looked good enough.

In theory you need a true CGA card to be 100% register compatible. I think the original version of Digger requires this.

However, some EGA cards come with a software utility to (allegedly) enable full CGA compatibility. Video 7 VEGA is an example.
EGA on a CGA monitor will give you 320x200x16 mode, so definitely worth having.

We played Digger (booter so I suppose it was the original version) on an Amstrad PC1640. That one had a Paradise graphics chip which could be set to CGA, EGA and Hercules using either dip switches or a software utilits (I guess it was native EGA). The only CGA game I remember any kind of problems was IIRC Moon Patrol. I think I had to adjust the monitor controls (maybe through keys in the game?) so the image wouldn't wrap around or cut off.
(I wrote about it in this thread: Paradise PEGA programming info (Amstrad PC1640)?)