EGA card with external oscillator - what does it do?

[Malvineous]

Hi all,

I recently picked up some ISA cards off eBay, and one of them was an "EGA Ultra" (Gemini VC-001 chipset) as shown at the VGA Museum.

However this particular card came with a "daughterboard" of sorts, which consists of an oscillator attached to what I am guessing is a feature connector. I found a pinout in this thread and it seems the board takes +5V from the EGA card, feeds it to the 36MHz oscillator, and takes the output back to the card on the "Ext Osc" pin.

So clearly the card is providing a 36MHz clock on a pin designed to accept an external oscillator, but I am not sure what this achieves. Is it to provide a picture on a fixed-frequency monitor, or increase the refresh rate, or what? I am guessing it can't have anything to do with changing the screen resolution as that would require software support.

Any ideas?

 

[ejs]

Neat! Looks like it is indeed providing an external clock (EXT OSC pin). It's also grounding the FEAT 1 pin (mapped to bit 6 of the STATUS 1 register), probably so that software can detect that the card is there. You can select this external clock with the Miscellaneous Output register (0x3C2) bits 3:2. 00=14.318MHz, 01=16MHz, 10=EXT OSC. I'm not aware of any software that used this feature...

 

[Malvineous]

Interesting! I've done some more digging and apparently you *can* use it to display higher resolutions, though by the looks of it only in text mode. Apparently the EGA BIOS supports redefining text modes so you can change mode 0 from 40 columns of text to 120 or more columns by using the external oscillator, as explained in this PC Mag article. Apparently it's compatible with any app that correctly uses the BIOS to query the card.

I'm guessing that's what this was used for, perhaps with some word processing application that was designed to handle this kind of modification.

I had no idea the EGA was capable of that!

 

[ejs]

Nice find. Looks like the article was written by Charles Petzold, how funny...

EGA is full of interesting stuff like that. Ever wonder what the two RCA jacks are for? They are wired to the feature connector, so presumably someone could make a little adapters for generating NTSC or high resolution monochrome video.

 

[Malvineous]

Oh really! I just assumed they were composite video outputs. I didn't realise you'd need a feature connector card in order to use them (although I did wonder why there were two!) Are CGA cards the same or do they by default put out a composite signal?

 

[modem7]

Oh really! I just assumed they were composite video outputs. I didn't realise you'd need a feature connector card in order to use them (although I did wonder why there were two!)

Some IBM EGA information at [here].

Are CGA cards the same or do they by default put out a composite signal?

The IBM CGA card outputs NTSC composite on its RCA jack.