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| Macintosh Plus video overview?? |
Posted by: mikemac on 2026-09-10 13:18:30
Does anyone know of an overview of how the video circuit on a Mac Plus works? Something a SW weenie might understand? I was looking at the schematics for the ITXPLUS and couldn't figure out how the VIDOUT signal was generated. Looks look PAL magic to me! Is the 68K involved in the video generation? Cycle steal PALs? Any clue would be helpful. |
Posted by: joevt on 2026-09-10 13:48:41
The 68K is not involved. I think somehow the framebuffer reads for the video out must be interleaved with the CPU RAM accesses since both are accessing the same RAM. |
Posted by: bigmessowires on 2026-09-10 14:46:41
The dedicated video circuitry shares access to RAM with the 68000, and operates during periods when the CPU is not accessing memory. It's a hardwired circuit that reads the frame buffer and generates the 1-bit video signal. I wrote an article about it 15 years ago (ouch!!!) when I was developing a Mac Plus hardware clone. https://www.bigmessowires.com/2011/08/25/68000-interleaved-memory-controller-design/ |
Posted by: mikemac on 2026-09-10 15:40:00
The dedicated video circuitry shares access to RAM with the 68000, and operates during periods when the CPU is not accessing memory. It's a hardwired circuit that reads the frame buffer and generates the 1-bit video signal. I wrote an article about it 15 years ago (ouch!!!) when I was developing a Mac Plus hardware clone. https://www.bigmessowires.com/2011/08/25/68000-interleaved-memory-controller-design/ Thanks! That will keep me entertained for quite a while! I'm trying to decide what I want to build next. Thinking of something like the ITXPLUS but hopefully simpler and using fewer, more modern parts. And yes, I don't know anything about the complexity of matching existing Mac HW yet. I'm still in the "ignorance is bliss" phase! |
Posted by: bigmessowires on 2026-09-10 17:35:20
There's an interesting continuum of possibilities with hardware clones.
On one end, you could make an exact replica of the original hardware, duplicating the original PCB with modern design tools, and populating it with new stock of the original chips when possible or else with old chips harvested from original machines. Some clever people here and elsewhere have already done this for many classic Mac models.
In the middle of the spectrum, you have projects like ITXPLUS that are roughly faithful to the original hardware, but with various changes, substitutions, and improvements to reflect the changing times (for example VGA output).
On the far end of the spectrum, you can recreate the entire computer inside a single FPGA. This is what my Plus Too project did (linked in the previous post), which is now part of the MiST / Mister ecosystem of vintage computer FPGA-based hardware-level emulators.
All of these are hardware clones which seek to duplicate the functionality of the original computer down to the smallest detail, as much as possible. This puts them in a different category than software simulators like Mini vMac, Basilisk II, and Sheepshaver, which provide a user experience that mimics the original but skip over a lot of the low-level hardware details. For example I believe Mini vMac does not attempt to simulate the floppy drive behavior at all - it just traps high-level filesystem API calls and then "magically" puts the requested sectors into the supplied memory buffer. One notable exception is MAME, which I think tries hard to be extremely faithful to the original hardware even though it is a 100 percent software-based emulator. |
Posted by: mikemac on 2026-09-10 20:53:40
Where to draw the line has always been the tripping point for my retro designs. That's why they mostly stay as figments of my imagination! 🙁
People can do amazing things with FPGAs. I'm not one of them! I hated Quartus and could never figure out how I was supposed to debug my design. It's a skill I wish I had but I don't think I have the time nor patience for learning it now. A big PAL (ATF1508) is about my limit.
In consumer 68K machines, one quickly runs into custom ICs which limits one's options. In the Mac world, it seems like the Mac Plus was the last one that didn't include custom ICs. So I was thinking maybe something a little more modern than the ITXPLUS. (I have more 68sec000 stashed away than plain ole 68Ks!)
Ultimately I'd like to build something comparable to an old Sun 3/80m with a 68060 running NetBSD, X, ... For no rational reason! |
Posted by: mikemac on 2026-09-11 14:07:39
Anyway, back to the Mac Plus video: can anyone think of a reason a bus snooper that implement a shadow frame buffer to VGA/LCD/HDMI/... using something like a RP Pico, STM32, ESP32, ... wouldn't work? That would eliminate a lot of PALs. |
Posted by: bigmessowires on 2026-09-11 15:45:45
Snooping the bus and capturing writes to the frame buffer memory should work. Rob Braun did this for a Mac SE several years ago, and it should be very similar on the Mac Plus. http://synack.net/~bbraun/sevideo/
But since the video signal coming off the motherboard is already a 1-bit digital signal, it may be a lot easier to just grab that signal (plus hsync and vsync) with a microcontroller, rather than bus snooping. Your micro can store the bits in memory, and then spit them out again on the next frame with appropriate pixel doubling and letter-boxing and timing changes to make a standard VGA signal. Check out https://github.com/guruthree/mac-se-video-converter |
Posted by: mikemac on 2026-09-11 16:44:43
Snooping the bus and capturing writes to the frame buffer memory should work. Rob Braun did this for a Mac SE several years ago, and it should be very similar on the Mac Plus. http://synack.net/~bbraun/sevideo/
But since the video signal coming off the motherboard is already a 1-bit digital signal, it may be a lot easier to just grab that signal (plus hsync and vsync) with a microcontroller, rather than bus snooping. Your micro can store the bits in memory, and then spit them out again on the next frame with appropriate pixel doubling and letter-boxing and timing changes to make a standard VGA signal. Check out https://github.com/guruthree/mac-se-video-converter I don't actually have a Mac Plus to grab the 1b video off of! I have a sad Centris 610! 🙁 The ITXPLUS uses a Pico like what you suggest. I'm trying to decide if designing my own version using a mc68sec000 and a CPLD (ATF1508) instead of all the small scale stuff on the ITXPLUS is feasible.
Given all of the discouraging info over on the Macintosh 4MB limit thread about how limiting the Classic Mac's address space is, it may not be worth pursuing. I'd go back to considering a 68060 Raven except, in a fit of stupidity a year or so ago, I threw out all of my ISA cards! 🙁 I guess I'm just not meant to play with retro stuff. |
Posted by: arroz on 2026-09-25 14:22:14
Hi!
Just a note, if you take the “grab the video signal” approach, I built a converter that grabs it and outputs DVI (HDMI): https://codeberg.org/arroz/MacDVI Feel free to use the code.
Regards
Miguel Arroz |
Posted by: mikemac on 2026-09-25 16:15:08
Hi!
Just a note, if you take the “grab the video signal” approach, I built a converter that grabs it and outputs DVI (HDMI): https://codeberg.org/arroz/MacDVI Feel free to use the code.
Regards
Miguel Arroz I thought I had made a deal on a SE motherboard but we could never finalize the sale. So I'm playing with a Performa 630CD instead using the built-in video. So I no long have a pressing need for grabbing the Plus/Classic/SE video off of the PDS slot. But I will bookmark your project if I ever have the need again!
Mike |
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