A Technical History of Apple's Operating Systems

On 7/28/2006 at 9:55 AM Don Y wrote: Well, consider that the probable reason for the "stop bit" was to allow the printing mechanism in a TTY to complete its cycle. If I remember my old Model 14 innards right, a start bit caused a clutch to engage and, as a rotor made its cycle, receipt of a mark pulse would cause one of several selector bars to move. At the terminus of the revolution, only a single typewriter-style typebar would be selected by the bars and it would print. The clutch then disengaged and the unit wated for the next start pulse. If you think about it, TTY-style async was a strange thing to keep around when the main application of the protocol was in more-or-less permanently connected setups. NRZ data would have made more sense in this situation--and one could have at least done away with stop bits--they're not really necessary for today's crystal-controlled bit rate clocks. But gosh darn it, we're still playing like we're moving those old selector bars when we have our equipment speak async. Cheers, Chuck .