Adapter to use internal compact Mac screen?

[marchie]

I have to believe somebody has made this before...

Is there some kind of adapter that lets a modern small board computer (like a BeagleBoneBlack or Raspberry Pi), send its video to a compact Mac CRT?

I have some good tubes, and some analogue boards that could be recapped, and all I REALLY want is a text based console to connect to my other systems.

 

[marchie]

Ok, I guess my coffee kicked in properly and I washed thru the AI soup of Google and found this.

I think this is absolutely viable for me.
I just have to make OpenBSD do it....

Control a Macintosh Classic CRT with a BeagleBone Black – Intro

Read how we started our Macintosh Classic CRT project.

nerdhut.de

 

[Durosity]

Ya know I was sure I’d seen a fairly cheap adapter that someone had made for a Pi for exactly this job, but for the life of me I can’t find it anywhere.

 

[dv-]

I know that some of the youtubers have hacked the video signal from the motherboard to a A/D converter. So I'd assume it's a relatively standard signal type.

Honest question here: is there a reason you can't just go the other way - take, say, the green signal line from an analog signal and route it to the logic board?

 

[cheesestraws]

Honest question here: is there a reason you can't just go the other way - take, say, the green signal line from an analog signal and route it to the logic board?

Do you mean "route to the analogue board" here?

If so, there's no particular reason it wouldn't work if you get the timings on the signals right (and it's the right voltage etc). That's quite a big if, though, as there wasn't really a standard for these signals at this point. But this is basically what all the greyscale options for the SE/30 do: they synthesise video waveforms with ... close enough timings to the signals generated by the logic board and just send those to the AB instead of the signals from the LB.

You do have to be quite careful though - it's possible to physically damage things if you get the parameters wrong here.

 

[dv-]

Do you mean "route to the analogue board" here?

If so, there's no particular reason it wouldn't work if you get the timings on the signals right (and it's the right voltage etc). That's quite a big if, though, as there wasn't really a standard for these signals at this point. But this is basically what all the greyscale options for the SE/30 do: they synthesise video waveforms with ... close enough timings to the signals generated by the logic board and just send those to the AB instead of the signals from the LB.

You do have to be quite careful though - it's possible to physically damage things if you get the parameters wrong here.

Yes, I meant route to the analog board. Sorry.

That's interesting. Thank you.

 

[zigzagjoe]

Difficult to call it standard. It's a video signal, but it's odd enough that commodity video hardware won't be expecting it.

Pixel clock is nonstandard (15.6672mhz) along with resolution (512x342)
Video is digital TTL signalling. (0 - ~4v) VGA is typically 0.7v peak into 75 ohms. Might be inverted too. Can't recall.
VSYNC is nothing unusual.
HSYNC drives the horizontal transistor directly, it's far longer than a standard hsync signal and extends into the display period (outputting pixels to screen) instead of just being active in the time between lines. Also, damage to the hardware is likely if anything goes wrong with this signal!

To use the Mac CRT with an external video signal, you could inject an external video signal in place of the LB signal so long as you exactly match the pixel clock of the LB and align your video signal's syncs with the LB's syncs. It's difficult to achieve if your (external) video hardware is not explicitly designed for genlocking (synchronizing the syncs). This is the approach I take with my grayscale card.

Otherwise, you need to generate the complete video signal+syncs yourself... not impossible but fiddly. That beaglebone article is a good overview. The pixel clock isn't a problem, it doesn't need to exactly match what the LB used. 16mhz is fine, for example. However, you will need a way to generate the nonstandard horizontal signal. This can't be done with commodity video hardware (ie. a analog video output off something) without additional logic. This is the approach Bolle takes with his grayscale card.

If you were trying to use an analog video signal, you may also possibly need amplification on the video signal with the stock neckboard since analog video signalling is typically lower voltage than TTL. Video also needs to be black and white (not grayscale or otherwise) as the stock neckboard doesn't expect anything but white or black and won't do anything sensible with an analog (shades of gray) video signal. This is why the neckboard must be replaced on Grayscale setups for SE/30.

The beaglebone guy is not using video hardware at all, he's using programmable logic hardware on the SoC so he's able to take the steps needed to generate the HSYNC, and the video signal is just a digital output pin which means logic levels are close enough to work. This approach could be replicated pretty easily with a Raspberry pi Pico's PIO hardware.