Getting external video output from a compact Mac

[Baldung99]

What I'm trying to do here is, getting some sort of output from a classic mac. I assume all of them follow a sort of standardized from of output to their internal CRT. I haven't been able to find anything concrete. It's probably some weird old format that was seen as perfectly normal for back then but now nothing will recognize. What kind of signal do those things output to and how do I convert it to something that modern (think, VGA) hardware can accept? What I'm looking to do here is, replace the internal CRT of a classic Macintosh with an LCD panel. I think some of the older ones (512K/128K or thereabouts) actually use the motherboard to drive the CRT circuits, so I wonder if that would be an issue.

I say a compact Macintosh, because I really haven't gotten my hands on one yet. I'm just trying to research which one would be the easiest to add an LCD panel to (and remove the CRT).

 

[cheesestraws]

What I'm trying to do here is, getting some sort of output from a classic mac

The book Build Your Own Macintosh and Save A Bundle is about building, among other things, desktop machines from compact Mac logic boards.  This obviously includes some discussion of how to get video out.

Another option: I think it was bbraun who managed to get an image of the screen via the PDS slot using a microcontroller, but the website seems to be down at the moment, at least for me, so I can't check which mac it was (I think it was an SE).

 

[blitter]

I assume all of them follow a sort of standardized from of output to their internal CRT. I haven't been able to find anything concrete. It's probably some weird old format that was seen as perfectly normal for back then but now nothing will recognize.

You assume incorrectly. The early Macs' video signal is designed for its analog board and nothing else. The closest analogue (pun unintended) to something contemporary with the Mac-- remember that VGA wasn't invented until 1987-- is the 350-line mode of EGA... and even EGA wasn't invented until *after* the first Macs shipped in 1984. The early compact Macs output 370 lines of video at a horizontal scan rate of 22.254kHz, of which 342 lines are visible. EGA tops out around 21.8kHz in 350-line mode which is *almost* what the Mac puts out. But don't misunderstand-- EGA and the compact Mac video signal are *not* compatible. bbraun's adapter works by bus sniffing the video RAM area of an SE with a fast stm32f4discovery board. You could probably adapt that to other compact Macs, though it'd be messier without the PDS interface.

The CRT *tube* in an early compact Mac *can* be swapped out for a monochrome tube with the same neck pinout. I've seen mods with the green-phosphor IIc tube, mods with an amber tube, and even a mod that went so far as to drive a 15" B&W TV.

 

[Kai Robinson]

There is a TTL mod you can do to output a 9-pin CGA style monitor, if that's useful?

 

[cheesestraws]

There is a TTL mod you can do to output a 9-pin CGA style monitor, if that's useful?

Yup, that's in that book I mentioned (or, at least, in the first ed.).  Is what I was thinking of, but I couldn't remember what kind of monitor it talked to.

 

[Trash80toHP_Mini]

I think that's a different adapter. IIRC these are from the book mentioned.







As noted on the PCB, it's a composite video interface. Lower right cable is the co-axial connection.

bbraun developed a far better interface, don't recall if he finished it?  ISTR it was a bit-bang of video memory technique that's a lot more efficient for putting the image on a modern display? Don't have the link handy to his development site.

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