There have already been some good descriptions of UARTs posted. I thought
I'd point out that they are derived from mechanical UARTs in teleprinters
like those made by Teletype Corporation. I used to use Teletype model 15
printers for ham radio. These used a 5 bit code (Baudot). The transmitted
data consisted of a start bit, 5 data bits, and 1.5 stop bits. A lot of
UARTs will still do 1.5 stop bits. The printer had a "selector magnet."
This magnet held in an armature when the line was in the mark condition,
which was the idle condition, with 60mA flowing through the coil (more on
this later). The armature kept a shaft from rotating. The shaft was driven
through a clutch from a synchronous motor driven off the AC line. Some
units had DC motors with governors. When a start bit came along (a "space"
condition), the magnet dropped out and the shaft was allowed to turn. At
various positions during the rotation of the shaft, the position of the
selector magnet armature was sampled. If the magnet was pulled in, a lever
was put in the mark condition. If it was released, a lever was put in the
space condition. In this manner, five levers were positioned during the
rotation of the shaft. When the shaft finished its rotation, the selector
magnet was pulled in for the stop bit, and the shaft would be stopped (the
clutch would slip). The five levers drove 5 "vanes" that went across the
front of the printer. These were pivoted in back. The front could be
either up or down for the mark and space condition of each of the bits.
Levers on the moving carriage transferred the position of these vanes to
slotted curved bars below the type basket. These bars also had one of two
positions for mark and space for each bit. One slot would line up on all
the bars when a particular character code was sent. This allowed another
bar that was attached to a particular type bar to fall into the slot. A
"bale" (I'm trying to remember all the terms we used for the parts of
these things) came forward during the stop bit and would catch the type
bar that had fallen through the slot, pulling it forward, and causing the
type to fly up and hit the paper through a ribbon, printing the character.

There were two types of selector magnets: Pulling and Holding. A Holding
magnet would have a cam push the armature up to the electromagnet just
before the sample time of each bit. The cam would then drop the armature.
If it stuck, there was current, and the line was in the mark condition. If
the armature dropped, it was a space bit. On pulling magnets, the cam did
not do that, so you needed higher current to pull the armature in.

If you used pulling magnets, the two coils were usually in series and
would run at 60mA for mark. If you used holding magnets, the coils could
be in parallel for 60mA or series for 20mA. On my 60 word per minute
machines, each bit was 22ms, and the stop bit was 31ms.

By the way, the terms "mark" and "space" came from Morse's original
telegraph, I believe. His original telegraph had a moving paper tape and a
pen. When there was line current, the paper would be marked. When there
was no line current, the paper would not be marked, causing a space. A
telegraph operator would read the marks and spaces (forming dots and
dashes) to read the telegram. However, telegraph operators were able to
read the telegram by just listening to the sound of it. So, the whole
thing was simplified by getting rid of the pen and paper, just giving a
telegraph sounder. Anyway, mark and space from the 1860s carries over to
UARTs today.



Harold


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