LCIII frame buffer capture from card in VRAM SIMM slot?

[Trash80toHP_Mini]

Notion here is that a capture card in the VRAM SIMM Slot of the LCIII could monitor/capture/make use of data as it is written into the onboard VRAM frame buffer setup. Dunno if it would need a mod to the board or not, I don't think VRAM is addressed as distinct banks like DRAM using CAS/RAS?

I'm thinking (hoping) it's more like straight address mapping as is the case with PSRAM and SRAM? If so, I'd think a capture card in the VRAM SIMM slot could provide a direct digital feed to an LCD Controller?

 

[SuperSVGA]

VRAM is mostly just DRAM if I understand correctly.
It also has SAM registers which are static and may be what you'd want to use to read the data, but I'm no expert on this.

 

[cheesestraws]

I suspect you'd have more luck snooping writes to VRAM addresses on the PDS connector, personally.

 

[Trash80toHP_Mini]

@cheesestraws thanks, hadn't considered that. Just remembered that a lot of add on video interface magic was done through memory interfaces. Would loading of the frame buffer'd data be read off the PDS more easily? Video Memory is sometimes buffered as in the IIsi, so I'm wondering if all the data needed could be read off PDS?

"The Macintosh LC III supports a 640 x 400 video mode that allows users to display 16-bit color on 13-inch RGB and VGA monitors."

So that's 640x400 @60Hz in 512K of VRAM implemented as a quartet of 256K x 4-bit ICs soldered to the main logic board. I imagine putting headers alongside the VRAM ICs and plunking the capture board on right there would leave the VRAM SIMM open for business. Only need single bit depth for this application, but switching to higher resolutions off the standard analog interface for external display only is in the plan.

 

[nickpunt]

@Trash80toHP_Mini another method could be if there's a video chip of some kind that can be piggybacked off of using a standard PLCC socket, like this Presenter 575 that grabs the video signal from LC 575's to put out to VGA/HD15:

 

[Trash80toHP_Mini]

Thanks you find the coolest stuff and apply them to the craziest of projects!

I'll be looking at the Devnptes again it seems. Have you(or anyone else) got one of those boards? Given the LCIII's remarkably flexible video output it's now front and center in my crazy quests. Expect a couple of surprises from maxx should things work out.

Meanwhile, there's a remote possibility of capturing output straight off the Analog Connector:

VGA->Arduino->Mac Portable LCD

Crazy notion of the day: Can this cute little puppy be trained to run in reverse? credit: https://dqydj.com/arduino-640x480-vga/ Goal would be capturing a 640x400 60Hz VGA compatible RGB feed for outputting 8bit digital input for the Luggable's LCD? Four years ago this was a crazy...

68kmla.org

 

[nickpunt]

Yeah that's my MacPresenter. I don't know if I ever properly tested it, I recall it being hard to get the PLCC socket to stick on the chip and I never got around to putting doublesided tape on it to just stick it on. Happy to send it off to someone to reverse engineer.

Looks like someone just sold one too. Don't see them around too often https://www.ebay.com/itm/203804354963

 

[bdurbrow]

If you've already got a system to do something with the video data once you have it, then it should - in principle - be not to difficult to build a declROM that tells the system to just put the data wherever you want it in address space. The basic Mac II setup is well documented, as you well know...

If you write an INIT/Extension to snoop some low memory globals to find where the main screen's buffer is in address space, grabbing writes to that space from the PDS slot should - based on my reading of "Designing Cards and Drivers for the Macintosh Family" - work as well; and it would be a lot cleaner than trying to snag data directly off of the VRAM chips.

The only downside to this approach, is of course, that it occupies the PDS slot.... so you might want to think about adding a buffered pass-thru slot to the board if there's enough room and and the timing margins are OK.

 

[Trash80toHP_Mini]

Aha! Thanks much, now I understand what @cheesestraws was suggesting. Not a worry in the world about using the PDS slot for snooping. Plenty of room on the logic board for whatever's needed as it's a new twist on an already reverse engineered board design.