Mark G. Forbes
|Andy's mention of the 'old pacing characters on XTs' reminds me
of where I first saw stop bits.
Back in the mid '60s through about 1982, the old Teletype KSR-33
was in widespread use. The advent of PCs pretty much wiped them out
within a few years. These were electromechanical keyboard/printer
terminals, As Seen On TV In Old Movies. They ran 110 baud (11cps)
and as I recall used two stop bits.
My recollection was that if you tried to use them with one stop bit,
there wasn't enough time between successive characters for the cam
assembly to get all the way stopped after each printing stroke, and
it would cause the output to get all messed up. These things ran on
a solenoid selector and cam arrangement that rotated and lifted a
cylindrical printhead. At the end of the character, a 'club' whacked
the printhead into the ribbon and made the impression on the paper.
The other thing I recall was the end-of-line handling, which as I
recall needed a 'null' character appended for my particular TTY. That
gave the printhead carriage enough time to return to the left hand
side of the platen before the next character arrived. That's why CR
and LF are separate characters; CR moved the carriage back, and LF
caused the platen advance solenoid to kick the paper up one line. If
you didn't add the null, the next character would print while the
carriage was still moving to the beginning of the line.
When I worked in the radio business, we had an even older teleprinter,
used for UPI wire copy. This one ran at 50 baud. Caps only, of course,
as were the KSR-33s. We also didn't have a lot of the nifty characters
in use today, (like backslash, tilde, etc.) but on the Unix systems of
the time, we made do with funny looking escape sequences. Alas, I don't
remember what they were, but you could go take a look at the source
for a TTY device under Unix, and you'd find it. Ah, the good old days....
Mark G. Forbes, R & D Engineer | Acres Gaming, Inc. (541) 766-2515
KC7LZD | 815 NW 9th Street (541) 753-7524 fax
peak.org | Corvallis, OR 97330 forbesm
"There has been an alarming increase in the number of things I know nothing
Prior to the KSR33 Teletype were the model 15, which I used
extensively, the model 14 "transmitter-distributor" (tape reader), and
model 14 "reperf" or "typing reperf" (tape punches). The typing reperf
typed the characters as well as punching the tape so it was human
readable. Prior to this was the model 12 which was actually a bit more
electrical than the model 15. The model 12 printer used a mechanical
distributor to do parallel to serial conversion and vice-versa. The
model 15 did it all mechanically. Quite amazing!
The model 15's I used were 60 words per minute. The start bit
and the five data bits (using Baudot code) were 22 mS long. The stop
"bit" was 31 mS long. This was CLOSE to the 1.5 stop bits many serial
ports can now do.
The speed was set by a synchronous motor (though some used
governor regulated motors). I believe the shaft would finish one
rotation in 154 mS (7 bit times). The transmit end would hold the line
in the mark condition for another 9 mS to allow for slight speed
variations between transmit and receive. During the stop bit, the
selector magnet would be pulled in and the armature would put a stop in
the way of the rotating shaft. When a lump in the shaft came around, it
would get stuck on the magnet armature, stopping the shaft rotation and
slipping the clutch. When the magnet was released during a stop bit, the
shaft was free to rotate again.
The model 15 originally ran 60 mA through series wired "pulling"
magnets. In the mark condition, the magnet would pull the armature to
the core. In the space condition, the armature would be released. Later
machines had "holding" magnets. A cam on the shaft would move the
armature up to the core just prior to bit sampling time. The cam would
then quickly drop away. If the armature stuck, it was a mark. If it
fell away, indicating no magnet current, it was a space. the use of
holding magnets allowed the loop current to be reduced to 20 mA for
series wired magnets, or 60 mA for parallel magnets.
These machines also used high voltage DC (about 150 VDC) to run
the magnets. You could get enough current with 12 VDC, but it took too
long to build up the current after a keyboard switch contact closure due
to the magnet inductance. Use of high voltage with a series resistor
resulted in a faster rise time when the keyboard switch closed (since
As someone else said... "the good ol' days?"
> Back in the mid '60s through about 1982, the old Teletype KSR-33
> was in widespread use. The advent of PCs pretty much wiped them out
> within a few years.
The world does not revolve around the PC. Nor did it then.
Inexpensive video terminals, first at 1200 baud then at the
unheard of 9600 baud, and dot matrix printing terminals had
essentially replaced the ASR/KSR-33, -35, and -37 (which had
lower case letters) by the middle of the 1970s.
The KSR was keyboard send/receive. What did ASR mean?
> These were electromechanical keyboard/printer terminals,
> As Seen On TV In Old Movies. They ran 110 baud (11cps)
> and as I recall used two stop bits.
110 baud is correct; 11 CPS is incorrect. Each character
was 11 bits (1 start bit, 7 data bits, a parity bit, and
2 stop bits). 110 baud / 11 bits/char = 10 char/second.
> My recollection was that if you tried to use them with one stop bit,
> there wasn't enough time between successive characters for the cam
> assembly to get all the way stopped after each printing stroke
answer to above quiz:
ASR was automatic send/receive. A paper tape reader/punch was
attached on the left side. If a prepunched paper tape was
loaded, an inquiry from the host computer would start it going.
Your could use the keyboard and paper tape punch to make tapes
without being connected to a computer. Vice-versa, you could
print tapes to the printer while off-line.
That's also why DEL has character value 0x7F in ASCII. It's
all holes punched in the paper tape. Literally a strike-out
character when you made a mistake in punching a tape off-line.
(Lots of DEL characters was a great way to make paper tape)
(chaff for those parties when I was in college. )
re: Old Dogs and TTYs ;-)
Mark, it's nice to know I'm not the only `old-timer' around here ;-)
- Tom (Portland, OR)
At 12:52 PM 5/30/97 -0700, you wrote:
>Andy's mention of the 'old pacing characters on XTs' reminds me
>of where I first saw stop bits.
>Back in the mid '60s through about 1982, the old Teletype KSR-33
>was in widespread use. The advent of PCs pretty much wiped them out
>within a few years. These were electromechanical keyboard/printer
>terminals, As Seen On TV In Old Movies. They ran 110 baud (11cps)
>and as I recall used two stop bits.
And back in the 12'th century, when I was commanding a viking ship out to
plunder Russia, we had a navigation system which we had to program by
putting wood sticks in different holes on a map. They were connected by ten
meters of twisted guts tied to a rocking chair, which was the brain of the
system. It took 25 men two weeks to figure out the position of the ship,
based on the direction of the whirls in a cup of ale in the center of the
rocking chair's wheel.
THAT was the good old days!
(Seriously, many thanks to the author for the posting about why stop bits
are used - it was excellent! When I was in "computer fundamentals" training
to become support technician at Gateway 2000 Dublin, none of the teachers
knew why stop bits were used (they didn't know very much at all))
All this talk about Teletype machines stirs memories. I was a
Sophomore in 1971 when I majored in Journalism and Broadcasting and got
involved with our campus radio station.
In the news room, was a Western Union Teletype machine which plodded
along at 60 WPM seven days a week. Remember that there weren't enough bits
to go around in the five-bit Bodot code so the printers had a Figures mode
and a Letters mode. The three rows of keys found on a Bodot sender either
sent UPPER CASE LETTERS AND A FEW punctuations in Letters mode, or more
punctuations and numbers in Figures mode. The shift of the S was the Bell,
for example and the top row of keys was either QUWERTYUIOP or 123456789
depending upon the mode. Some Teletypes automatically reset Letters mode
after each new line while others just stayed in whatever mode they were last
left. It wasn't uncommon to miss a character and if that happened to be
a mode change, there was a dead-tree version of a lookup table kept in a
handy drawer next to the machine in case one needed to salvage a bit of copy
that got mistranslated.
You could tell when the tape sender in Oklahoma City either
malfunctioned or when a breaking story occurred because the printer would
stop its rata-tat-tat and fall silent except for the hum of the motor. It
would then begin to haltingly type out a message that was obviously being
hand-sent at the time. Soon, the normal rhythm would start up again and
you would know that the tape reader was back on line.
The terminal box that connected the telephone line to the Teletype
machine had a neon lamp which probably sat right across the loop and glowed
as long as there was voltage on the line. I remember thinking at the time
that a person could probably receive the data by holding a photo cell against
the signal lamp.
As an amateur radio operator, I understood basically how the system
worked and thought it was kind of interesting to actually see
such a system in operation.
In 1972 or so we got really modern when a KSR33 in all its massive
bulk was installed to link with NPR Headquarters in Washington. That printer
was full ASCII and screamed along at about 10 letters per second. I don't
know for sure, but I think it probably had a bipolar loop supply to make those
magnets move fast enough. It was driven by a telephone modem so all the
high voltage was locally generated. It was deafening when it began to print
and it tended to drown out all other sounds in the room. Thankfully, it
tended to print a page or two and then fall silent for most of the time.
I remember wondering how much faster electronic printers would ever get.
I never even dreamed that we would have what we do now.
Martin McCormick WB5AGZ Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group
I forgot to mention in my last posting about the old Western Union
news wire machine that when the loop was interrupted such as when a back
howe cut the cable, the machine would chatter as if it were receiving data
because the system was receiving a continuous space condition. You could
hear it running a little faster than normal because it wasn't having to
synchronise for the next start bit.
The clutch was always in and there was nothing to stop it.
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