• Please review our updated Terms and Rules here

Has anyone built a device that will convert the signal from the display port on the back of the IIc to an MDA/HDA/CGA 9pin connection?

CompaqGuy1993

Member
Joined
Mar 31, 2022
Messages
26
I want to use an amber Smith Corona word processor monitor with 9pin MDA connection from the 90's with my old IIc and am looking for a device to buy or build that will make this possible. Any help is greatly appreciated.
 
There's no reason that this couldn't be done, but I'm not aware of a device that exists already, off the shelf.

Anything is possible, but there are things that make this non-trivial.

There was a device called the "Color Enhancer" by Video7 which was an adapter to hook a digital RGB monitor, IE, CGA. But to be clear here, CGA is *not* the same as MDA. CGA monitors run at the same 15khz horizontal refresh frequency as the IIc's NTSC output, so this device only has to assemble suitable RGB signals out of the raw data available on the IIc's "video expansion port". (That connector on the back of the IIc spits out a raw copy of the signals that feed into the NTSC composite generation hardware; it wasn't really strictly meant to be used for RGB, the Apple peripheral that used it was that rare-as-hen's-teeth LCD panel.) The issue with an MDA monitor is that it runs at a different, incompatible horizontal frequency, and they also run at 50Hz vertical frequencies instead of 60hz NTSC.

Some MDA monitors will tolerate running at NTSC frequencies but others can actually be damaged if you try. So unless your Smith Corona monitor is happy running at NTSC frequencies you'll either need to incorporate a scan converter or hack your IIc to output more friendly sync... which given that Apple II's are highly timing-reliant computers and all their clocks are based off the video timing chain would probably be a non-trivial task. Scan conversion is a highly fraught process when the target monitor runs at a different vertical refresh from the source material, because the process is going to involve digitizing whole frames and playing them out asynchronously, which is both "hardware expensive" and prone to producing nasty visual artifacts like tearing.

There are VGA adapters to go on the IIc's video output, here's an example that includes fun features like emulating a green monitor. The nice thing about these devices is because the VGA mode they upscale to is also running at 60hz they can generally avoid serious problems with artifacts. (And it's also helpful that VGA's horizontal sync is an even twice as fast as NTSC, meaning simple line-doubling techniques work.)
 
Also the PAL modulator/adapter devices. I assume the A2M4020 version (said there to convert NTSC video to PAL) must also convert a 60 Hz video signal to a 50 Hz signal.

Yes, per above, that webpage is on crack/confused. The PAL modulators were intended to be plugged into a PAL IIcs, which already were set up for 50hz video, and the difference between the two model numbers is one is *just* an RF modulator for the native PAL monochrome signal generated inside a PAL IIc, and the other one does PAL color encoding. Which is why the A2M4020 only spits out mono video when plugged into a PAL IIc. If you plugged it into an American IIc then presumably it would happily modulate the NTSC signal, which a dual-standard TV capable of locking onto would display in color. (It's actually pretty common for PAL-region TVs from the late 80's and later to accept NTSC signals, even if they don't explicitly say they support them. It's a shame that doesn't go the other way.)

In any case, even if you laid hands on a PAL IIc for its 50Hz video frame rate it's still a different Hsync from MDA. It's in fact almost identical to NTSC. (15,625hz vs 15,750hz; this is why the crystals used for EU-standard computers are generally only a tiny bit different from the US versions, the line rate is almost identical.) MDA's 18.43 kHz is *way* off. The matching frame rate would help with tearing but you'd still have to design your converter as a full frame-capture async device.
 
Well, better not refer to the A2M4020 for your 60 Hz to 50 Hz scan conversion then. Oh well.

Digging around apparently the US RF modulator had the same part number. I’m wondering now if there was any difference besides the label. (My ignorant assumption was that an RF modulator for PAL would use a different frequency band than an NTSC one, but I honestly don’t know if that’s true. It would make a certain amount of sense for TVs made worldwide to be able to use the same tuner components.)
 
I want to use an amber Smith Corona word processor monitor with 9pin MDA connection from the 90's with my old IIc and am looking for a device to buy or build that will make this possible. Any help is greatly appreciated.
Open it up, many late MDA screens had an oscillator and pot adjustments, a few ram at 15khz as they weren’t intended for MDA
 
Open it up, many late MDA screens had an oscillator and pot adjustments, a few ram at 15khz as they weren’t intended for MDA

It wasn't uncommon for late-era digital mono monitors to be dual-standard and support either MDA or CGA, but when I say "not uncommon" I wouldn't say "common" either, and it seems pretty unlikely one included with a typewriter would be particularly "fancy".

If the monitor in question is a Smith Corona HRM2 it's spec'ed as MDA frequencies, and a photo of its connector plug I found on eBay shows only sx wires, which indicates it doesn't support CGA. (It only has the pins for HSYNC, VSYNC, Intensity, mono video, and two grounds; notable in their absence are R, G, and B, which are present if the monitor is one of the CGA-emulating variety.) I've also seen pictures of these monitors displaying video, and it's definitely not NTSC frequency.

While it's certainly *possible* with enough fiddling that you might be able to beat this thing down to accepting NTSC frequencies it's not going to do it without some hurting.

Assuming you *did* go through the trouble to do that then... *shrug*? Looking at this it looks like the expansion plug doesn't have separate horizontal and vertical sync signals, only a combined composite sync. You'll probably need to separate that to drive a monitor of this type. Once you'd done that... it might be as simple as hooking SEROUT to the video in signal? (Although... maybe you'll have to combine that with combine that with VIDD7... and maybe WNDW* to get a fully sensible output?) The result will be simple mono, no shades of gray.

There really isn't much to be gained here because if you have the color killer enabled the mono composite output from a IIc will be almost as clear as a digital out if you feed it into a monochrome NTSC monitor.
 
Last edited:
Thanks for the feedback. I'm certainly no expert on video signals and what it takes to make them work between connection formats, so a project like this is probably out of my reach for the time being unless I can dig up a step by step project someone else has already done.
 
@CompaqGuy1993 the way to do this would be to use a sync converter and color stripper to convert the CGA frequencies to MDA compatible ones, but you're taking professional video equipment there. It can certainly be done, but I agree with @Eudimorphodon in that there are good ways to get what you want without going that far.

- Alex
 
Back
Top