A couple of months ago I lamented the fact that historic USB documentation appears to have vanished from the face of the Earth. Today I finally found one such document, the USB 0.9 specification from April 13, 1995, published almost exactly nine months before the final USB 1.0 specification.
And where did I find the USB 0.9 specification, you may ask? Why of course, on my own hard drive. Not kidding. It’s been there the whole time. I really hate when that happens!
Anyway, back to USB. Back in December I complained that the Wikipedia article on USB claimed that USB 1.0 only specified the 1.5 Mbps low-speed transfer rate, and USB 1.1 added the 12 Mbps full-speed transfers. I also wrote that I had a vague memory of 12 Mbps being in fact defined first, and 1.5 Mbps added later, but I could find no evidence for that. Either way, USB 1.0 most certainly defined both speeds.
To recap, the March 20, 1995 issue of InfoWorld reported that “USB supports a 1.5MBps data transfer rate—compared to a standard serial port data transfer rate of approximately 400KBps”. A letter from the USB program manager at Intel published in the April 17, 1995 issue of InfoWorld stated that “USB supports a 12Mbps data transfer rate”. Note the capitalization.
In retrospect, it is apparent that InfoWorld simply screwed it up. When they wrote 1.5MBps for USB, they really meant 1.5 megabytes per second, which translates to 12 megabits per second. Except when they were talking about serial ports, they surely meant 400 kilobits per second.
After reviewing the USB 0.9 specification, I can say with certainty that the hints of low-speed USB transfers being a late addition are correct. The USB 0.9 specification defines only one transfer rate, 12 Mbps. There isn’t the slightest mention of any other transfer rate. That means the 1.5 Mbps low-speed transfers were probably added in the USB 0.99 specification in August 1995 (and yes, I checked, the USB 0.99 spec is not on my hard disk).