Unpopulated mystery header on slotloaders?

[Trekintosh]

Noticed this unpopulated header while playing with the logic board on my G3, here's a picture with it highlighted from wikimedia:

My guess is an unpopulated mezzanine slot or similar, but does anyone have any actionable info? Could be fun to hook something up to it...

 

[Nixontheknight]

Probably, they did axe it on Rev C or D iMacs, and one of those revisions still had the header, so it was very late in development that they axed it

 

[Oelmuvun]

Don't have anything important to add, just some side notes. Some 1999 machines had DVD decoder(? It's been a long time, fuzzy memory) plugged in on the battery-side, but not sure about the one you circled.
Some pictures in Apple's service manual show a similar header along side the Down Converter Board populated near where you have circled.. Though the board you have pictured looks like the later version and slightly different layout.

 

[Trekintosh]

Probably, they did axe it on Rev C or D iMacs, and one of those revisions still had the header, so it was very late in development that they axed it

Makes sense.

Don't have anything important to add, just some side notes. Some 1999 machines had DVD decoder(? It's been a long time, fuzzy memory) plugged in on the battery-side, but not sure about the one you circled.
Some pictures in Apple's service manual show a similar header along side the Down Converter Board populated near where you have circled.. Though the board you have pictured looks like the later version and slightly different layout.
View attachment 26982 View attachment 26983

Mmm ok so that means that smaller unpopulated header must also be a PCI connector of some sort, pretty sure that's the only way a decoder board could work, unless it's like... PC Card or something, which is just PCI anyways. I'll investigate that one too at some point.

Good find on the populated mezzanine socket in the SM though. Hard to tell but it sure looks like the same connector as the Rev A/B imacs but with some kind of dummy plug in place. Maybe the early rev imacs had it too and they've all been lost to time?

I found some of the Molex 52760-1609 that are said to be the mezzanine slots for cheap and I'll solder one on just to try it. Still need to find the mating connector, there's a surprisingly large variety of them.

Someone on reddit posted pics of their DVD decoder board. I might try and find one of them just for fun.
https://www.reddit.com/r/VintageApple/comments/w3y71i

 

[Trekintosh]

I found the Columbus (iMac g3 tray) schematics which have a pinout of the mezzanine slot. I’d be surprised if they changed between boards, but next time I have a chance I’ll probe the power and grounds that I can find on my slot loader to confirm they’re at least in the same spot.

then I’ll make a relatively simple board that breaks out the pci lanes into usb 3 sockets so I can use usb 3 cables to break into pci slots for testing.

 

[Trekintosh]

If any mods see this can you move this to the hardware subforum? Thanks.

 

[Trekintosh]

ARgh! Am I ever a bozo. I even went and ordered connectors! Somehow I never comprehended until now while staring at the picture above that this "mezzanine slot" was 140 pins, while the original was 160 pins. The existing pinout is genuinely worthless for reverse engineering this. What a maroon. I don't have the chops to reverse engineer a pinout from scratch without a higher end logic analyzer than my poor old HP 16500C. Even then, I'd be limited to blindly probing until I found address lines on a running iMac. Yikes.

None of the surrounding chips can be beeped out either, since I don't have any pinouts for them.

Without a schematic there is absolutely no way I'm capable of making this slot actually do anything, so I don't suppose y'all have any slot loading iMac schematics lying around, do you?

Edit: I might be able to borrow the Firewire daughterboard from an early iMac DV and use the firewire chip (publicly available, TI TSB12LV23) to map out the pin assignments on its header, so this isn't a complete wash. I want my PCI iMac G3