Or, Yet Another Wikipedia Mystery
Looking for something on the Wikipedia USB page, a detail caught my eye. In the version history section, both the table and the text claim that the original 1996 USB 1.0 specification defined 1.5 Mbps low speed transfer rate, and the 1998 USB 1.1 update added the much faster 12 Mbps full speed. I would like to say that that’s not how I remember it, but the truth is that I had no trouble completely ignoring the existence of USB until 2003 or so. Anyway, it certainly did not match what I had read elsewhere.
Finding authoritative information about the prehistory of USB is remarkably difficult for something that happened in the (early) age of the Internet. But finding a copy of the actual USB 1.0 specification is not that difficult. Sure enough, both full and low speed is defined there, so Wikipedia once again proves a questionable source of information. What mystifies me in this case is not so much how it’s possible that Wikipedia is wrong (with so much information, some of it is bound to be wrong) but how this particular error crept in. It’s something that is reasonably easy to verify, but more importantly, why would anyone even think that only the 1.5 Mbps low speed was originally defined? It’s not just a typo, it’s very specific misinformation.
I have a vague memory of reading somewhere that it was in fact low speed (1.5 Mbps) that was added to USB later, while full speed (12 Mbps) was the original speed. But either I’m remembering wrong or it happened before USB 1.0 was finalized; as mentioned above, information about USB prehistory is not easy to come by.
USB 1.0 full speed was capable of transfers in the 1 MB/s range. It was intended for floppy drives, printers, scanners, and similar devices. Low speed is much much slower and due to limitations on the USB packet size and transfer period, low-speed devices can only transfer a few hundred bytes of data per second. The advantage of low-speed devices is that they’re cheaper and use less power (slower controller chips), and the cables can be thinner (cheaper, more flexible) and longer because twisted pair conductors or shielding is not required. In other words, the USB 1.x low speed was meant for input devices such as keyboards and mice which need long and flexible cables but need not transfer much data.
The USB 1.1 specification in reality brought very few changes. Most notably there was no new USB 1.1 host controller hardware, the existing UHCI and OHCI controllers could handle USB 1.1 devices just fine. The updated USB specification largely clarified various details that were less than obvious in the USB 1.0 specification.
Note: Any pre-1.0 USB specification would be very welcome. It is clear that the USB specification goes back to at least 1994, but nothing from before 1996 appears to have survived, at least publicly. At least the USB 0.99 specification was public.
Apr 2022 Update: In the meantime, the USB 0.9 specification has been located. It is dated April 13, 1995. It never uses the terms full-speed or low-speed. But it clearly talks about them:
The standard USB cable will consist of one pair of 20-28 AWG wire for power distribution with another 28 AWG pair twisted, with a shield and overall jacket. This will be used for typical peripherals operating at the rated 12 Mb/s signaling.
An alternative cable of identical guage but without the twist or shield is under investigation. This will be used in a sub-channel proposal applicable to cost sensitive interactive peripherals and will be described in revision 1.0 of the specification.
In all other respects, the mechanical specifications for the sub-channel will be identical to the fully rated specification.
The “sub-channel” devices were clearly low-speed USB. The USB 0.9 specification confirms that the “full-speed” 12 Mbps USB was indeed specified first. In April 1995, low-speed USB was mentioned (though not by that name) but not defined except for cabling. The 0.9 specification in several places says things like “the data rate is 12 Mb/s” and it clearly was not fully updated to account for low-speed USB. There is no hint as to how fast the “sub-channel devices” might operate.