I am aware about loop area and emissions (EMI considerations), but it seems you are saying that the deliberate design decision by Apple engineers to break the motherboard ground plane into 2 pieces -- 2 pieces connected to each other only by that single trace -- "lowers emissions from the chassis" by way of the very slight resistance and parasitic inductance inherent to that single trace, allowing that single trace to somewhat act like a very low end ferrite bead, filtering HF noise coming into the motherboard from the chassis (or connectors).
The one they they probably do get, is less current across (laterally) the connector area -- though I'm still not very confident in this because of the sheer number of traces crossing the gap. To the extent that the outer metalwork and metallization are imperfect, this could help to reduce the voltage drop between them, as seen from the outside (where it matters, to EMC) -- it just seems a very cut-off-your-nose-to-spite-your-face kind of move.
Right, which is why I don't really understand why the broken up ground plane is needed at all. Just keep the entire ground plane ground, with no breaks, and add ferrite beads, etc. at the noise points, which would be where the metal bracket solders to the motherboard, and of course the connectors, which are all on the same edge of that motherboard.
Maybe they read one of those trade mags back in the day, discussing slotted grounds, and got way too excited about the trick.
I see it all the time; far more often than is deserved. 99.5% of the time you think about slotting a plane -- no, it's actually worse that way, don't do it.
It's those tiny few cases remaining, where it does help out. But knowing which ones, requires a lot of experience.
No idea who built this board, how much experience they had -- maybe it was intentional and expertly done, maybe it was ignorant, we can't tell at this point. (Unless you get lucky and they're on Twitter and watching, or something.
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To reduce inline image width, per your suggestion, I tried adding "[img width="500px";]http://" & [img width=500px]" to no avail. Any further specifics would be greatly appreciated.
...Where at? I don't see an edit...
Like this (quote my post to see the code):
Also, fortunately the forum shrinks images to fit width, so those of us using modestly sized browser windows don't have to strain our eyes to look at things. Also, you could consider sparing a few megabits on your viewers by reducing the images in the first place.
It is plain old SCSI-2 on the SE and SE/30 computers.
Ah, so parallel it is (I guess? Wikipedia still isn't terribly specific, at a glance..), and that explains the lack of terminators.
The 4 "FILTER" chips are BOURNS RC networks, part number 4120R-601-250/201. They are becoming harder to find, so someone has made a recreation here.
Ah, cool! So they do get some attenuation there, that helps a lot.
9" B&W CRT displaying 512x342 pixels.
So without porches, and at 60Hz, 10.5MHz pixel clock; shouldn't be much higher than that, actually, I would be shocked if they have much overscan... maybe they put a 2/3 PLL in that system chip or something? Which I guess is handling graphics too, and, hmm would it have onboard VRAM for that then? At least being monochrome it doesn't need a whole lot I guess.
PCs were much simpler in that respect; graphics was either a pile of chips around a CRTC (6845 usually), or some ASIC doing the same things (probably most EGA+?). The attached VRAM of which is very obvious.
(My XT clone indeed has 128k Paradise EGA; what luxury! Too bad the ISA bus is so dreadfully slow, along with the 8086, to do very much with it...
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Tim