weird external video adapter in my Mac Classic...what display is this for?

[KennyPowers]

This "card" is in my Mac Classic:



It plugs into the power/video/sound connector on the logic board and passes it through, and then there's a (quite long) female DB9 cable plugged into it that someone has cut a hole in the back of the case for:




I scoped this thing and traced out the circuit, and it's definitely an external video adapter. I just have no clue what kind of display it's meant to drive. The 9pin connector on the back of the case outputs a positive 22kHz horizontal sync signal on pin 8 and a positive 60Hz vertical sync signal on pin 9. Pin 4 is the monochrome digital video signal. Those pin choices are similar to CGA/EGA, but the sync rates don't match up. They almost match EGA mode-2, but then the vertical sync would have to be negative and mine is positive. Anyone know what kind of display this could have been for?

 

[GiGaBiTe]

There's only a quad input NAND gate on that board, so the most I'd imagine it would be doing is inverting some part of the video signal and leaving the rest alone. So it'd more or less be the internal 512x342 resolution at 60 Hz.

I wouldn't really call it a "video adapter" because it's just intercepting the video signal from the logic board and breaking it out to an external port.

Maybe it was designed to be used with some sort of projector, but whatever it was had to accept the weird display resolution that nothing else used. Maybe a multisync monitor or a really old B&W terminal screen could sync up with it. Else, you'd have to use some sort of scan converter.

 

[KennyPowers]

There's only a quad input NAND gate on that board, so the most I'd imagine it would be doing is inverting some part of the video signal and leaving the rest alone. So it'd more or less be the internal 512x342 resolution at 60 Hz.

I wouldn't really call it a "video adapter" because it's just intercepting the video signal from the logic board and breaking it out to an external port.

Maybe it was designed to be used with some sort of projector, but whatever it was had to accept the weird display resolution that nothing else used. Maybe a multisync monitor or a really old B&W terminal screen could sync up with it. Else, you'd have to use some sort of scan converter.

Yes, when I traced out the circuit, the quad NAND chip is used to invert both the horizontal and vertical sync signals. So I guess the sync signals from the logic board are both negative because they both scoped as positive at the external port. The quad NAND is also used to invert the video signal from the logic board twice (so just buffering it to 5V), and then that buffered video signal is run through a 12ohm resistor before being routed to the external port. I don't know enough to know what purpose the resistor serves.

It's interesting that you say projector. I got this machine from my dad, whose garage it sat in for at least 20 years, and he doesn't remember much except that he got it second-hand. He thinks he may have bought it from a place that resold a lot of used tech from local schools and universities. I wonder if maybe this was for hooking up a projector so the class could see what the teacher was doing on this machine. I was under 10 years old though when this would have been used, so I don't know anything about what kinds of projectors were available at the time (were there "TTL projectors"?).

 

[GiGaBiTe]

It's anybody's guess as to what type of external display was used, a CRT projector just makes the most sense. I remember when I was in elementary school in the early 90s, we had these massive CRT projectors on roll around carts that were used to connect Apple II and Macintosh LCs to project video on a pull down screen. I don't remember all of the inputs they had, but the input section had at least a dozen or so different types of inputs.

Apple didn't offer any sort of official external display support on any of their early compact macs, or expansion options much at all for that matter, which lead to weird undocumented devices like these. There were hundreds of small companies offering weird expansion devices for the early crackerbox Macs, and the ones that offered external video almost never had a unified standard.

The only two things I could suggest to try today would be a scan converter (like OSSC) or an early multisync CRT that accepts TTL signals. You could also try one of those chineseium "Arcade" converter boards designed to convert weird formats into VGA, but those things can be a headache to get working.

 

[twolazy]

Guessing its a variant of this

Compact Mac video adapter « waveguide.se

[img=mac-se-video-bmva.jpg] For many years I have had an old Macintosh SE logic board that was scrapped out from an old Apple repair shop. Someone had broken the SIMM sockets at some point by snapping the retaining clips at the ends. A common problem u...

www.waveguide.se

Looks like it allowed to mirror the internal display instead of just robbing it for external video.