As I understand it, in the early days of personal computers, strict FCC limits on RF emissions, were a factor limiting the speed at which data could be transmitted over wires, but I'm trying to understand exactly how that worked. In particular, I came across the following surprising comment at http://atariage.com/forums/topic/239749-improving-800xl-sio-transfer-rate/

"The 400 and 800 were designed before the relaxed FCC rules, so SIO was bandwidth limited to work with 19.2 peripherals."

This referring to the SIO bus used to connect disk drives among other things to the Atari 8-bit computers, and 19.2 being in kilobits per second.

That is actually a very severe restriction, less than 3 kilobytes per second, much slower than the maximum transfer rate of a 5.25" floppy disk itself.

But the Apple II accessed disks at 15 kilobytes per second. How was this possible without running into the same limit?

Also, the Atari 800 was released in 1979. As I understand it, the new FCC rules came into effect in 1980, and were more restrictive rather than more relaxed (at least for class B, devices marketed for use at home, which would certainly include everything Atari sold); famously, the original TRS-80 generated so much RF interference that you could make up for its lack of sound hardware by putting it next to a radio and writing an appropriately crafted loop to generate the right pattern of interference to play music on the radio, but that model had to be discontinued at the end of 1980 for that reason.

So what were the actual limits on data transmission speed before and after 1980? And why exactly was Atari limited to 19.2 kbits/sec?

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    Still waaayy faster than the Commodore 64's ~4kbit/s
    – tofro
    Jul 31, 2018 at 20:41
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    @tofro True, but the C64's speed limit was for different reasons (bad decision to require backward compatibility with Vic-20 disk drives, plus a hardware bug discovered at the last minute on a schedule that left no time to fix it properly) unrelated to RF emissions.
    – rwallace
    Jul 31, 2018 at 20:44
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    People were happily enduring the C64 speed, which was at least 4 times slower than the Atari (for known reasons) - So, you can't really say the Atari speed would have been perceived as slow and call it a "severe limitation" - That was my point.
    – tofro
    Jul 31, 2018 at 20:51
  • @tofro I wouldn't say we were exactly happy about the speed of the 1541 :) But I take your point that the Atari speed would not have been considered a showstopper, which makes it more plausible that it could have been made faster by spending slightly more money.
    – rwallace
    Jul 31, 2018 at 20:54
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    "famously, the original TRS-80 generated so much RF interference [...] putting it next to a radio" I had a [clone of] the TRS-80 and using a radio was an occasionally useful debugging technique: you could learn to distinguish "running normally" noises from "stuck in an infinite loop" noise.
    – TripeHound
    Aug 1, 2018 at 8:00

2 Answers 2


So what were the actual limits on data transmission speed before and after 1980?

There were no limits, at least no artificial and regarding the transmission speed. The only limits were set by technology and effort (aka money) spend to reach a certain speed.

Transmission speed on corporate data lines was already at or beyond 64 kbps in 1980. No hassles there. So speed wasn't a concern.

All limits set by FCC (and other regulatory bodies around the world) were about radio emission, which were supposed not to interfere with other equipment and not to disturb legal (licensed) transmissions.

Radio interferences are defined by frequency and power used. If the power is low enough to not disturb standard equipment within usual distance, no regulation. Similar for frequencies, but since next to the whole spectrum is allocated, it again falls back to the power dissipated.

So if at all, it was about the way interfaces are build and shielding. There is (next to) no radiation when using adequately shielded cables and conenctors - just think HP-IB of 1960, specified for up to one MHz without any FCC trouble. Similar Centronics' parallel printer port. There's a reason these cables had shielding and metal covers on all connectors.

So if at all it's a question about what price they could charge for cables.

Bottom line, next to all these claims are rather unfounded and hot air ventilation.

the original TRS-80 generated so much RF interference that you could make up for its lack of sound hardware by putting it next to a radio

Same was true for the Apple II, as it was also lacking all shielding, but it got sold way into the 1990s. The II and IIe never got any shielding until 1993. The discontinuation of the TRS-80 was rather thru its comparably high production cost while at the same time being notoriously unreliable. But that's a different story.


45MBits (a DS3) were not really a challenge any more over copper wires (for Telcos) in the 80ies. So, provided you had the money, speed was not a technical problem.

As Raffzahn writes, you could (even back then) build data transmission equipment with much higher frequencies, provided you used appropriately shielded cables - which were obviously more expensive than non-shielded cables. Another method is to use parallel transfer with alternating signal and ground lines for shielding (that's what the Centronics printer standard did), or even differential pair connections that dampen interferences even more.

According to Wikipedia, a computer that included a TV modulator was technically considered by the FCC and other authorities "TV equipment" and fell under different rules as other home electronics - Something Apple avoided by not including a modulator.

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    "The Apple computers connect the drives via a parallel bus" - not realy. While it was a cable with several wires (20), the data transmission was serial - as it was the direct read or write signal. Transmission was near 250 kbps, so more than 10 times as the Atari SIO - and quite noisy.
    – Raffzahn
    Jul 31, 2018 at 20:11
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    @Raffzahn Just learned something, thanks. Lots of wires somehow automatially implied parallel transfer to me
    – tofro
    Jul 31, 2018 at 20:18
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    IBM PC floppies also connect via ribbon cable, but still transfer serially on a single line for read and a single line for write ...
    – dirkt
    Jul 31, 2018 at 21:02
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    @tofro: At Midest Gaming Classic, a computer designer from that era (I think it was Joe Decuir, inventor of some classic Atari machines) said that (IIRC) TI went so far as to design a system using a fiber-optic cable [i.e. one with no electrical connection whatsoever] to connect their computer to a separately-powered RF modulator in the hopes of avoiding being classification as "TV equipment", but then discovered that they'd still get classified that way anyhow.
    – supercat
    Jul 31, 2018 at 21:22
  • @supercat That's also mentioned in the linked Wikipedia article
    – tofro
    Jul 31, 2018 at 21:26

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