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Open Source 68-Pin 512kb VRAM
Posted by: Stephen on 2021-09-21 19:04:52 I've released the schematics, gerbers, and PCB files for a VRAM version based on the Apple 820-0605-A 512kb VRAM (GitHub). If you appreciate this project I encourage you to star the GitHub repository and share it with friends.
Posted by: mmu_man on 2021-09-22 04:17:08 Hmm why only PDF for the schematics? The "source" in Open Source means source, not a screengrab of it 😛
Posted by: mmu_man on 2021-09-22 04:25:49 Also, putting zip files in an SCM kinda defeats the purpose of it, you can't diff it (yes, with git it's even possible to visually diff gerber files)… But for that you need to have them directly in. It will also make the git repo smaller since git will only store the text diff between versions, not the whole zip each time. Anyway, nice project!
Posted by: Stephen on 2021-09-22 07:49:15 Hmm why only PDF for the schematics? The "source" in Open Source means source, not a screengrab of it 😛 The project was designed in EasyEDA. Thanks for your feedback! edit: EasyEDA source JSON file is in the "gerbers" directory and the first paragraph of the repo's readme has a link to the open hardware source, which includes access to the schematic sources. 👍
Posted by: mmu_man on 2021-09-22 16:47:32 Ah ok so the source is available but on some cloud instead of the git repo. At least it's public. Let's just hope the cloud doesn't vanish 🙂
Posted by: Stephen on 2021-09-22 19:55:47 Ah ok so the source is available but on some cloud instead of the git repo. At least it's public. Let's just hope the cloud doesn't vanish 🙂 Is there something particularly grievous about the project I've shared? Your responses sound very snarky! The EasyEDA source files are in the repo. There are tools to convert the EasyEDA files to other formats (e.g., KiCad). Because I do not use other tools I cannot confirm the accuracy of conversions, and as a result I have chosen not include converted formats. If anyone would like to submit converted formats I would gladly upload myself or approve PRs!
Posted by: Iesca on 2021-09-22 22:03:23 I've released the schematics, gerbers, and PCB files for a VRAM version based on the Apple 820-0605-A 512kb VRAM (GitHub). If you appreciate this project I encourage you to star the GitHub repository and share it with friends. This is awesome! Would be neat to also have a 256k version for those machines (like the LCIII, perhaps others?) that can only take the smaller of the two.
Posted by: mmu_man on 2021-09-23 03:04:32 Is there something particularly grievous about the project I've shared? Your responses sound very snarky! Oh no it's not about you or your project, I want it to be useful to the most people, I just genuinely don't trust online services to be available for eternity so I was concerned about long term. Sorry if that sounded mean. The EasyEDA source files are in the repo. There are tools to convert the EasyEDA files to other formats (e.g., KiCad). Because I do not use other tools I cannot confirm the accuracy of conversions, and as a result I have chosen not include converted formats. If anyone would like to submit converted formats I would gladly upload myself or approve PRs! I don't have KiCAD installed on my new laptop yet but I can have a try on the old one.
Posted by: olePigeon on 2021-09-23 07:55:23 Excellent work. I like seeing new homebrews. Would love some GWorld RAM, too. Seems to be unobtanium.
Posted by: Stephen on 2021-09-23 09:17:03 I don't have KiCAD installed on my new laptop yet but I can have a try on the old one. Appreciate it! Excellent work. I like seeing new homebrews. Would love some GWorld RAM, too. Seems to be unobtanium. Do you have more information I can refer to? Could be interesting. This is awesome! Would be neat to also have a 256k version for those machines (like the LCIII, perhaps others?) that can only take the smaller of the two. I believe the 512k is backward compatible with 256k; only the first half of the memory gets mapped. Admittedly, I don't know for certain.
Posted by: olePigeon on 2021-09-23 11:49:17 @Stephen It's a special 1MB 100ns 68-pin SIMM. Despite what some websites claim, it is NOT the same type of RAM as a IIfx. They are not compatible. I wish I could offer you more information, but the reality is that you'd need to get your hands on an actual 1MB SIMM for an 8•24 GC video card. Since they're so rare, I doubt anyone would want to donate one to be reverse engineered. 🙁
Posted by: trag on 2021-09-24 12:06:45 I've released the schematics, gerbers, and PCB files for a VRAM version based on the Apple 820-0605-A 512kb VRAM (GitHub). If you appreciate this project I encourage you to star the GitHub repository and share it with friends. Thank you for sharing. This is a nice project. Have you considered variations for different widths of VRAM chips. They seem to come in three different widths.
Posted by: Trash80toHP_Mini on 2021-09-24 13:31:07 Nice project, has anyone looked through the Designing Cards and Drivers series for Apple specs on the various memory interfaces? DevNotes are also good sources, I found the full spec for the PowerBook 100 memory modules in its DevNote. I'd imagine VRAM and GWorld RAM would be spec'd somewhere in the developer docs as well?
Posted by: mmu_man on 2021-11-23 18:17:03 I don't have KiCAD installed on my new laptop yet but I can have a try on the old one. Ok so I wasted a month on a new laptop to end up sending it back because it had so many issues… I just tried with the online conversion tool but the result misses all the ground planes. Not so surprising since it gave many warnings during the conversion: `Warning: unsupported shape RECT Warning: SOLIDREGION with arcs/circles are not supported yet!` So not really usable yet in KiCAD. Might still be a good test case if I ever want to write EasyEDA import directly into KiCAD.
Posted by: micheledipaola on 2021-11-29 03:46:24 I believe the 512k is backward compatible with 256k; only the first half of the memory gets mapped. Admittedly, I don't know for certain. I am really no expert but I guess the more the memory size, the higher the resolution / screen colors you can get as output... so why going for a lower one? I think even the first LC model can have a 512k vram... doesn't it?
Posted by: Siliconinsider on 2021-11-29 04:05:53 It's a matter of bandwidth more than capacity. If you have 512kB but only have the bandwidth to read 256k in the period you refresh the screen, your extra 256k is useless. Recent Quadra are using many 256k SIMMs in parallel to increase the bandwidth and access the higher resolution/color modes, something you could not do with 512k SIMMs as the bandwitdh would be halved. The useful bandwidth of a single VRAM channel does not allows for more than ~768k to be used, so this is why 1MB SIMMs were never made (and also why the LC III and cie can only use up to 768k). Further details here
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