Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!shadooby!mailrus!purdue!haven!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn)
Newsgroups: comp.unix.questions
Subject: Re: What do all the different parity options mean?
Keywords: parity space mark
Message-ID: <11771@smoke.BRL.MIL>
Date: 8 Dec 89 02:25:56 GMT
References: <4090@amelia.nas.nasa.gov> <1038@maxim.erbe.se>
Reply-To: gwyn@brl.arpa (Doug Gwyn)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 33

In article <1038@maxim.erbe.se> prc@erbe.se (Robert Claeson) writes:
>Some weird protocols uses two or even one-and-a-half stop bit.

1.5 stop "bits" are standard at 110 bps.  This has historical origins
in the design of electromechanical teletypewriter units such as the
Teletype model 33 (a real classic).  In asynchronous communication,
a UART receiver can be set for 1 stop bit even if the transmitter
provides more than 1, because the extra "stop" marking is
indistinguishable from line idle and won't confuse the receiver.

The basic operation of electromechanical teleprinters in this regard
is as follows:
	Current flows continually while in an idle condition.
	This current energizes a solenoid in the receiver that
	keeps a clutch disengaged.
	When a start bit occurs, the solenoid disengages and
	the clutch engages, typically locking a cam-like device
	onto a continuously rotating shaft.
	As successive data bits occur, a data electromagnet
	moves back and forth, setting up mechanical "gates"
	(selector levers or equivalent) that will determine
	the exact positioning of the print head.
	After all data bits have occurred, the rotating cam
	triggers the print mechanism and the selected character
	is printed (along with carriage advance, etc.).
	The stop bit energized the solenoid that catches and
	holds the rotating cam with open clutch, restoring the
	idle condition.
If you like mechanical devices, these machines are fascinating.
Rumor had it that Teletype Corp. had their engineer Kleinschmidt
committed to a mental institution and would let him out just long
enough to design a new model.  From studying these creations, I
can believe it!