4 ms·

A lot of modern hobby computer projects use a FPGA, micro-controller, or dedicated modern CPU to bit-bang the video signal. How did the original 8-bit computer

by erik 7y ago

A lot of modern hobby computer projects use a FPGA, micro-controller, or dedicated modern CPU to bit-bang the video signal. How did the original 8-bit computers do it? Did they have access to chips that are no longer produced?

randrews 7y ago
Exactly. :)
jacobush 7y ago
Yes, they had.
duskwuff 7y ago
The Apple I and Apple II both used discrete logic to generate a composite video output. The Apple I used a couple of unusual Signetics parts (2504, 2513, 2519) and a character generator ROM to implement a text-only "video terminal" with a UART-like interface; the Apple II used a more complex system built around the system's DRAM refresh which allowed for graphical output. None of this required any truly unique parts (although the Signetics parts used in the Apple I are hard to find nowadays), but most hobbyists prefer something more modern than composite video. :)
- simcop2387 7y ago
  It's also a problem using discrete components for noise and EMC compliance, you need a shielding because you have a lot more higher frequency traces compared to otherwise. It's still possible to do without too much trouble but compared to doing most of it in a single devices there's just very little point in spending that much work most of the time.
  - duskwuff 7y ago
    EMC compliance isn't really a concern until you're building a consumer product. A breadboard kit… nah. :) Noise is a real issue -- but the main concern there is the CPU. Video generation isn't unusually noisy.
    - wizardforhire 7y ago
      “EMC compliance isn’t really a concern”... Unless you’re this guy https://hackaday.com/2019/05/15/the-great-ohio-key-fob-mystery-or-honey-i-jammed-the-neighborhood/ https://hackaday.com/2019/05/15/the-great-ohio-key-fob-myste... But seriously, you’re totally right.
LIV2 7y ago
Most used the 6845 CRTC or compatible but the Apple 1 used discrete logic & shift registers instead of CRTC & RAM from my understanding I don't think the particular shift registers etc are still available but it is still possible to do it with discrete logic these days I guess different projects have different focus and some probably don't care too much how they get video as long as it works. I've built discrete and CPLD based video circuits for my own 6502 project which is fun but took months to get working perfectly
derefr 7y ago
Enlightenment may come from looking at an IBM-PC MDA (Monochrome Display Adapter) board: http://www.minuszerodegrees.net/5150_5160/cards/ibm_mda.jpg http://www.minuszerodegrees.net/5150_5160/cards/ibm_mda.jpg ...or a VT100 main-board: http://i.imgur.com/Xae4be6.png http://i.imgur.com/Xae4be6.png 80% of these boards are "the graphics part." Dozens of chips! This is what the sibling commenters mean when they're saying that the video signal is "discrete logic" (as opposed to integrated logic.) It'd be a bit of an ask to have a hobbyist build one of those themselves (adding an 800% complexity increase to what is otherwise a very approachable hobby project.)
- tyingq 7y ago
  Well, and an exact replica would output composite video, which wouldn't work with anything most people already have.
ChuckMcM 7y ago
http://www.swtpc.com/mholley/RadioElectronics/TV_Typewriter.htm http://www.swtpc.com/mholley/RadioElectronics/TV_Typewriter.... was one of my favorites (the TV Typewriter) Seriously though, as amusing as they are, and as much programming as I did on a 16 line by 64 character "terminal" screen, I really could never go back to that. For grins I set up an xterm that way just to experience it and yeah, it still hurts. A lot. Even the 80 character by 25 line experience (although tolerable for many things) feels so very cramped these days.

by erik 7y ago

randrews 7y ago

Exactly. :)

jacobush 7y ago

Yes, they had.

duskwuff 7y ago

The Apple I and Apple II both used discrete logic to generate a composite video output. The Apple I used a couple of unusual Signetics parts (2504, 2513, 2519) and a character generator ROM to implement a text-only "video terminal" with a UART-like interface; the Apple II used a more complex system built around the system's DRAM refresh which allowed for graphical output. None of this required any truly unique parts (although the Signetics parts used in the Apple I are hard to find nowadays), but most hobbyists prefer something more modern than composite video. :)

simcop2387 7y ago
It's also a problem using discrete components for noise and EMC compliance, you need a shielding because you have a lot more higher frequency traces compared to otherwise. It's still possible to do without too much trouble but compared to doing most of it in a single devices there's just very little point in spending that much work most of the time.
- duskwuff 7y ago
  EMC compliance isn't really a concern until you're building a consumer product. A breadboard kit… nah. :) Noise is a real issue -- but the main concern there is the CPU. Video generation isn't unusually noisy.
  - wizardforhire 7y ago
    “EMC compliance isn’t really a concern”... Unless you’re this guy https://hackaday.com/2019/05/15/the-great-ohio-key-fob-mystery-or-honey-i-jammed-the-neighborhood/ https://hackaday.com/2019/05/15/the-great-ohio-key-fob-myste... But seriously, you’re totally right.

LIV2 7y ago

Most used the 6845 CRTC or compatible but the Apple 1 used discrete logic & shift registers instead of CRTC & RAM from my understanding I don't think the particular shift registers etc are still available but it is still possible to do it with discrete logic these days I guess different projects have different focus and some probably don't care too much how they get video as long as it works. I've built discrete and CPLD based video circuits for my own 6502 project which is fun but took months to get working perfectly

derefr 7y ago

Enlightenment may come from looking at an IBM-PC MDA (Monochrome Display Adapter) board: http://www.minuszerodegrees.net/5150_5160/cards/ibm_mda.jpg http://www.minuszerodegrees.net/5150_5160/cards/ibm_mda.jpg ...or a VT100 main-board: http://i.imgur.com/Xae4be6.png http://i.imgur.com/Xae4be6.png 80% of these boards are "the graphics part." Dozens of chips! This is what the sibling commenters mean when they're saying that the video signal is "discrete logic" (as opposed to integrated logic.) It'd be a bit of an ask to have a hobbyist build one of those themselves (adding an 800% complexity increase to what is otherwise a very approachable hobby project.)

tyingq 7y ago
Well, and an exact replica would output composite video, which wouldn't work with anything most people already have.

ChuckMcM 7y ago

http://www.swtpc.com/mholley/RadioElectronics/TV_Typewriter.htm http://www.swtpc.com/mholley/RadioElectronics/TV_Typewriter.... was one of my favorites (the TV Typewriter) Seriously though, as amusing as they are, and as much programming as I did on a 16 line by 64 character "terminal" screen, I really could never go back to that. For grins I set up an xterm that way just to experience it and yeah, it still hurts. A lot. Even the 80 character by 25 line experience (although tolerable for many things) feels so very cramped these days.