wiretap wrote on 2021-02-24, 19:38:

This is a very cheap project ($10 - $20 after shipping), and easy to build. You may even have all the necessary parts on hand if you have protoboard laying around. Obviously you'll need a standard EGA graphics adapter and monitor to use this.

The idea of this adapter is that the EGA core chip can take an external pixel clock (actually also kind of like a system clock for the EGA board) from the feature connector, so all you need for your 120-column text mode (or 960 pixel graphics mode) is to select the external 24MHz clock instead of the built-in 16.257 (or 16.000 on some clones) clock, and programming the horizontal timing for 50% more characters. This is indeed a very easy thing to do, but I'd expect that this will in fact run older EGA cards out-of-spec, so don't take it for granted it works on all "standard EGA boards".

It should work on all EGA boards that use chips or chipsets that were designed for use in Super EGA cards. The clock for 800x600 often was nearly 40MHz on those, so running the chips at 24MHz is a piece of cake - if the RAM is fast enough.

I won't swear it works on all Super EGA cards, too, as some of them might have ditched the external clock from the feature connector in favour of an additional internally generated clock. For example my Genoa-based "Expert Top SEGA 800" supports 14.318MHz (for 200-line modes), 16.257MHz (for 350-line modes), 26.824MHz (for 480-line modes) and 39.000 MHz (for 600-line modes). This leaves no clock select bit combination for "external clock". I just buzzed it out: On that card, pin 28 from the feature connector, used to input the external clock, is shorted to one of the outputs of the 26/39MHz dual clock generator (possibly through the wire bridge soldered at "J7"). Plugging the Super EGA-120 into that card is a recipe for disaster. OTOH, you don't need an external 24MHz clock if you have an onboard 26MHz clock for 132 character modes. The incompatibility with some Super EGA cards thus doesn't really matter.

Yes, of course. It is use at your own risk, read the article, and know what hardware you have.

I'll also note, on my KEA EGAmate-132 that they have the FEAT0 and FEAT1 pins bridged to ground. This is a 132-column adapter (26MHz clock) that came on my Diamond Flower EG-1000 EGA graphics card. I haven't looked a whole lot into the feature connector functionality, but maybe bridging FEAT0 and FEAT1 to ground forces the external clock without software selection or needing other jumpers on the video card? The EG-1000 has no external clock select jumper.

wiretap wrote:

but maybe bridging FEAT0 and FEAT1 to ground forces the external clock without software selection or needing other jumpers on the video card

FEAT0/1 are outputs from the adapter back to the EGA chip. In another board, it was used to detect the presence of the board by software. The mode change needs to be handed by software. There aren't jumpers for the clock selection, at least in the EGA boards I've seen. There are usually switches for the default mode though, read by the EGA BIOS at boot. So grounding it kind of makes sense in a simple design.

mkarcher wrote:

For example my Genoa-based "Expert Top SEGA 800" supports 14.318MHz (for 200-line modes), 16.257MHz (for 350-line modes), 26.824MHz (for 480-line modes) and 39.000 MHz (for 600-line modes). This leaves no clock select bit combination for "external clock". I just buzzed it out: On that card, pin 28 from the feature connector, used to input the external clock, is shorted to one of the outputs of the 26/39MHz dual clock generator (possibly through the wire bridge soldered at "J7"). Plugging the Super EGA-120 into that card is a recipe for disaster.

Yes, I found that out by accident on my Genoa. Which is kind of an annoying design choice. Adding another onboard clock by using the EXT OSC line, essentially breaks the purpose of the feature connector. They should have left off the feature connector on these boards.

the3dfxdude wrote on 2021-02-25, 18:48:

wiretap wrote:

but maybe bridging FEAT0 and FEAT1 to ground forces the external clock without software selection or needing other jumpers on the video card

FEAT0/1 are outputs from the adapter back to the EGA chip. In another board, it was used to detect the presence of the board by software. The mode change needs to be handed by software. There aren't jumpers for the clock selection, at least in the EGA boards I've seen. There are usually switches for the default mode though, read by the EGA BIOS at boot. So grounding it kind of makes sense in a simple design.

My ATI EGA Wonder has jumper P7 to select between internal oscillator or external oscillator on the feature connector. Table C-2 here: http://www.minuszerodegrees.net/manuals ... 01988).pdf

wiretap wrote on 2021-02-25, 20:24:

My ATI EGA Wonder has jumper P7 to select between internal oscillator or external oscillator on the feature connector. Table C-2 here: http://www.minuszerodegrees.net/manuals ... 01988).pdf

Great. It looks like they cared about compatibility in their card design. Genoa didn't. Too bad the EGA Wonder was out of my price range when I initially searched for an EGA card. I never bothered reading the specs closely.

KEA Systems EGAmate-132 gerber files attached.. just in case someone wants a clone of this 132-column mode adapter.