Trivia, uncovered by further yet mainly fruitless websearching, or maybe not so trivial if you are specifically looking for one is they were supposedly rebranded by Atari and distributed with their PC-1 computers. In addition they seem to have been part of a Haas industrial system controlling a CNC or something.

BitWrangler wrote on 2021-08-13, 02:48:

So does anyone know of it's full capabilites, do we just get MDA/Herc and CGA or does it do EGA mono too? I thought I had a monitor stashed that did CGA mono, but I thought it was a different one, because all I remember doing on this one was MDA/Herc.

Does the CRT have a captive video cable with all 9 pins populated? That's a good sign that the display supports both MDA and CGA.

You can also measure the resistance between GND (pins 1-2) and the RGB inputs (pins 3-4-5) as a quick way to see if they are connected to anything inside the display.

Roughly half of my MDA displays have turned out to also display CGA video in 16 glorious monochrome shades.

(I haven't tried the 350-line EGA modes on them but I doubt it would work.)

Thanks yeah I have a feeling it does have all 9 on it, I'll see if I can tease out the cable when I'm back there tomorrow. Elsewhere I was reading that there's a problem if it's grounded on pin 2 for anything other than MDA/Herc, to the extent of card damage. Also I was reading that certain adapters have 15khz EGA modes for compatibility with CGA monitors. Will have to find that ATI Small Wonder or whatever it was that does just about everything. I was last messing with it on a Turbo XT with onboard graphics for which there appears to be no info, so didn't wanna mess with the switches while it was working. Though when that comes out again, I'll see if the motherboard is known elsewhere.

kdr wrote on 2021-08-14, 01:40:

Roughly half of my MDA displays have turned out to also display CGA video in 16 glorious monochrome shades.

Be careful with this one, though: At least in theory monitors can be damaged by an unexpected horizontal frequency. Anecdotal stories claim that the original IBM MDA monitor and exact clones can be damaged in practice.

mkarcher wrote on 2021-08-14, 07:44:

Be careful with this one, though: At least in theory monitors can be damaged by an unexpected horizontal frequency. Anecdotal stories claim that the original IBM MDA monitor and exact clones can be damaged in practice.

Yep, the IBM 5151 MDA display had no horizontal oscillator at all, it just applies the sync input directly to the horizontal output transistor... so if you feed it a horizontal sync with a low enough frequency it'll probably destroy something in the horizontal output stage. (Presumably by letting the voltage and/or current in the horizontal deflection coils rise to dangerous levels.) I doubt that the 15.7khz sync of CGA is low enough, although I'm not going to test that on my own IBM 5151 just yet. 😀

In fact I think one of my MDA+CGA monitors *also* lacks a horizontal oscillator. As a result the raster scan is much wider in CGA mode, but because CGA has a lot of horizontal blanking ("overscan") and MDA doesn't have much at all, the final outcome is that both modes produce an active display of the same width.

Multifrequency monochrome CRTs (and combo MDA/CGA graphics cards that could automatically switch between MDA and CGA modes) were definitely a thing in the Turbo XT era. And seem to be common enough that it's absolutely worth checking the R-G-B pins to see if you've got one of them.