Article 4397 of comp.dcom.telecom:
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Date: 3 May 93 10:24:18 GMT
From: Jonathan_Welch <JHWELCH@ecs.umass.EDU>
Newsgroups: comp.dcom.telecom
Subject: History of Mark and Space
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Pat, I thought the following article might be worth reposting in
TELECOM Digest.  The original message was posted into a notes
conference on a vax DECUS members may subscribe to.  Once a month
tidbits from various notes conferences are reposted to the outside
world.


Jonathan Welch  VAX Systems Manager  Umass/Amherst  JHWELCH@ecs.umass.edu

                              -----------
DECUServe Journal, May, 1993   Beginning at page 15
(09/22/90 Harvey:  Mark and Space)

    "Mark" and "space" are curious terms to find in a hardware topic
discussing data communication.  They seem more appropriate for the
Windows conference.  But they are truly electrical communication
terminology and have many related forms, such as "steady mark",
"continuous spacing" and a seemingly unrelated term: "running open".
 
   I thought their origin might be of interest and along the way,
we'll discover where that curious "break" key came from that many of
us have on our keyboards and these days often use to get the attention
of the terminal server.

    These terms are very old and originated with an early graphical
device.  People never think of the telegraph as graphical
communication, but that's the way it was originally conceived.  Our
impression of the telegraph comes largely from movie stories of times
a century ago, when telegraph operators listened to the strange ticks,
tocks and rattles from the telegraph sounder and converted them into
urgent messages that pushed the plot forward.

    Morse didn't invent it that way.  His original device was an
electromagnet that pulled a pen (possibly a quill) against a moving
strip of paper.  When current flowed through the electromagnet, the
pen touched the moving paper and made a mark.  When the current was
off, a spring retracted the pen and there was a space on the paper.
Short marks were called dots.  Long marks were called dashes.

    Now this explanation is so simple and pat, it just has to be
largely legend and over-simplification.  There were many different
schemes, such as keeping the pen in contact with the paper and moving
it sideways by the electromagnet.  But the mark/space concept seems to
have stuck, because it appears in very early communication literature.

    This graphical device was actually used in production communi-
cation for a while. Some of the operators of the machines found that
they could recognize the "call letters" of their telegraph office when
the electromagnet and pen started tapping out a message on the strip
of paper.

    If the message was for another office, they didn't need to get up
to see if the message was for them.  Soon, they were able to just
write the message down on the telegraph form as it came in without
needing to "read" the tape.  When the operators were able to fully
"read" Morse code with their ears, they could stop putting ink in the
pen.  The telegraph sounder was born.
 
   You couldn't see the marks and the spaces between them anymore, but
they were still there in the minds of the engineers designing
telegraph systems.
 
   For good electrical engineering reasons, telegraph offices were
wired in series.  At one end of the railroad (for example) there was a
powerful battery with one pole connected to a rail and the other
connected to a wire that ran on posts for the length of the railway,
where it was also connected to the rail.  This constituted a simple
series circuit with the battery current flowing through the wire, into
the rail at the far end, and back through the rail to the battery.
 
   At each telegraph office along the line, the wire was cut, brought
into the office, sent through the coil of the electromagnet of the
sounder, then through the telegraph key, then back up to the pole and
on down the line to the next office.

    But you may have noticed a problem.  The telegraph key is normally
an open circuit.  When the operator pressed down on the key, the
circuit was closed and the current flowed.  How, then, did the current
flow when everything was hooked in series and all those keys were open
circuits?

    If you've ever looked closely at a real telegraph key, you may
have noticedthat it has a knife switch build into it, and that switch
is arranged to short the contacts of the key.  When the operator was
not actually sending a message, he or she (many early telegraph
operators were women) would close the knife switch so that the key
contacts were shorted and the whole series circuit was unbroken.
 
    Thus the normal idle telegraph line was in a "steady mark"
condition - a current flowed through all the sounders which if the pen
was still there would have caused a mark to be made on the moving
strip of paper.  The knife switch on each telegraph key was perhaps
the first "push to talk" button.  The operator had to "open" the knife
and break the circuit so the key could turn the current on and off and
send a message.
 
   Not surprisingly, this knife was called the break switch. When an
operator opened the knife the current stopped flowing in all the
sounder electromagnets and they went tock.  Everyone up and down the
line knew someone was about to start sending a message. The break
switch alerted them.

    When the Indians cut the telegraph wire, the circuit was open and
all the sounders went tock.  "Open" meant trouble.

    The graphical device didn't disappear, however.  The interest in
having the message automatically recorded on paper that could be read
without having to learn the arcane art of "reading" Morse code by ear
remained.  The inventors worked to improve on the simple marks
separated by spaces and actually make letters and figures appear.
 
    One early attempt was the telautograph.  It attempted to servo the
up/down and sideways movements of a pen being used to write a message
in longhand to a remote pen reproducing the motion and hence
re-creating the longhand.  It worked well for very short distances but
they didn't have the technology to send the control signals useful
distances.  There were other schemes using many wires.  Expensive.

    The big winner was the stock ticker.  It was the ancestor of all
the various asynchronous communication gadgets we have today.  It was
a triumph of mechanical ingenuity that enabled an ordinary telegraph
wire (and there were many) to be converted to actually print a message
in letters and figures on that moving strip of paper.  You didn't need
an expensive telegraph operator hanging around to "read" Morse and you
didn't have to puzzle out the strange patterns of marks and spaces.
But the communication technology was telegraph and the marks and
spaces were still there in the minds of the engineers.

    The stock ticker used the same series circuit technology of the
telegraph.  The wire ran from the floor of the exchange to the nearest
broker's office, through an electromagnet in the ticker machine, and
then on to the next office.  And yes, if the Indians (or a cleaning
lady) broke the wire anywhere, all the tickers went dead.

    Dead?  No, they went crazy.  The continuous telegraph current when
there were no stock trades being reported kept the ticker mechanisms
idle.  Steady mark.  Good.  The start of a trade message was a break
in the circuit (start pulse) which caused the ticker mechanism to
start spinning.  The following sequence of marks and spaces caused the
mechanism to select a particular character on its wheel and a hammer
struck the paper strip against it.

    When the circuit was broken by the cleaning lady, it was in a
"continuous space" condition, causing all the ticker machines to spin
their clockwork, "running open" until someone fixed the break. These
terms stayed with communication technology to the first minicomputers.
The venerable ASR 33 Teletype, one of the foundation stones of the
minicomputer industry, used telegraph series current loop technology,
marks and spaces, and "ran open" when you disconnected it from the
PDP-5.

    Well, if you got this far, you're probably wanting to know about
where your break key came from if you haven't figured it out already.
Yep, it's that knife switch on the side of the telegraph key.  You
didn't know you're a telegrapher, did you?


[Moderator's Note: Thank you for passing along a fascinating story. PAT]



