In article ,
TeeAye wrote:
With the original Baudot transmission over wires, a pen was used to make
a "mark" on a piece of paper (hence the name), and the "space" was, of
course, no mark. Given that terminology, I'd think the "space" was the
resting mode and "mark" was the active, or non-resting, mode. It's all
semantics though, anyone is free to define it any way they like.
An interesting, if irrelevant, discourse into history. :-)
If I recall correctly: these systems were set up as closed loops,
with a single current source, and with all of the transmitters and
receivers in series. The normal, resting state was to have a current
flowing through the loop, "marking" the recording medium. The active
state was to temporarily break the link, creating a "space" on the
recording medium.
This method had two advantages:
- It provided positive confirmation at all times that the loop was
intact. A broken wire would be noticed immediately even if there
was no active traffic on the wire, as it would cause all of the
recorders to drop out.
- One station could interrupt another station's transmission (to e.g.
send emergency traffic) by pressing the BREAK key on the
transmitter (or holding down the Morse sender key for a few
seconds). This would interrupt current flow in the loop, causing
all of the recorders or Morse sounders at all stations (including
the one currently sending) to lift their pens or stop chattering,
thus signalling the sending operator to cease transmission.
The terminology survived into the ASCII era, and there are still
traces of it in UART manuals to this day: the BREAK button on a
standard ASCII terminal creates a condition known as a "long space" by
changing the state of the transmission line from its resting state to
its active state for substantially longer than one normal character
time.
--
Dave Platt AE6EO
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