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The Apple II provided the ability to connect to your TV (as opposed to purchasing a dedicated monitor). However, at that time, the FCC criterion for applying stringent RF emission limits was 'device that connects to your TV' (as opposed to in the eighties when it changed to 'device that is marketed to consumers'), and Apple was unable to comply with those limits, so they dodged the regulation by encouraging another company to supply the RF modulator: https://en.wikipedia.org/wiki/Sup%27R%27Mod

Why was Apple unable to comply with the limits when e.g. Atari managed it? To some extent it was simply a matter of difficult engineering, but I have also seen it suggested that the expansion slots were the spoiler. I can understand how this might be so, given that the case had openings in the back through which cables could run to expansion cards. Later, the IBM PC solved the problem by covering all such openings with metal slats.

Was that the problem, and could the Apple II have solved it that way? Or was it a more fundamental problem of being unable to guarantee emissions compliance with every possible expansion card? Which way did the FCC see it?

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    This answer by Stephen Kitt on the design of the Atari 8-bit line (and some of the answer's comments) might shed some light: retrocomputing.stackexchange.com/a/2814/4222 – Jim Nelson Jan 17 at 4:38
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    The IBM PC solved the problem by being certified as a Class B device, not a general Part 15 Type I device. – Stephen Kitt Jan 17 at 8:54
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    It's a good question but because of the way computer makers used to worry about whether their product would pass FCC certification, I think much of the difference came down to nothing more than intuition, guesswork, and luck. – snips-n-snails Jan 17 at 12:56
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    @snips-n-snails some of that guesswork was vindicated by FCC rejections though, and subsequent relaxation of the rules (see for example the TI-99/4, at least according to this interview of Joe Decuir). – Stephen Kitt Jan 17 at 13:47
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    Could it be money? To get that FCC certificate you have to pay, even if you fail the test. So if you can work around by having a product that clearly doesn't need to be certified, you have one thing less to pay. I don't know about FCC, but a lot of these certificates have to be renewed if something is changed, and sometimes after a certain time, even if nothing were changed. – UncleBod Jan 18 at 14:33
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Why was Apple unable to comply with the limits when e.g. Atari managed it?

Purely through engineering that Apple was unwilling to carry out.

I have added a relatively detailed explanation of this to the Atari 8-bit article on the wikipedia.

The long-and-short is that it wasn't the slots themselves that were the problem, but providing some sort of external connections for those slots. Any openings in the case would be an immediate RF leak point. I am aware of only two cards that ultimately did have an external connection, an 80-column card and a Centronics printer card, which required routing through the case.

The other issue was that the shielding made airflow difficult, so the cards were limited in what they could do. I doubt that would have been difficult to fix had they desired it.

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The Apple Super Serial card did come with a metal back plate for the DB25 connector, which was clamped to the interior conductive coating on the plastic cases of later revisions of the Apple II+ and the Apple IIe.

IIRC, circa 1981, the Apple II+ with Disk drives and a Super Serial Card plugged in was tested for compliance with FCC Part 15 RFI/EMI regulations, when used with a shielded serial cable (which many hobbyists likely didn’t use).

So expansion slots were not fundamentally incompatible. But it did take a few years before Apple made that available in revisions of the Apple II that included coated plastic, conductive seals, and back plates for the expansion board connectors.

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