
Restoring a Teletype Model 15 - petethomas
http://www.aetherltd.com/refurbishing15.html
======
Animats
That's my site.

I restored five Teletype machines, and they were at major steampunk
conventions for almost a decade. Here's our telegraph office.[1]

I know about the guy at the Computer Museum, although I've never heard from
him. He built one of my interface boards. I built an interface that drives one
of the old teletypes from a USB port. It's entirely powered from the USB port,
even though it has to deliver 120V at 60mA.[2] He's also using my "baudotrss"
software.[3] He's much more into YouTube than I am. I never dreamed that
anyone would want 12 hours of Teletype restoration videos.

These machines were designed to be maintainable, at three levels. First, the
big components are hot-swappable. Remove two thumbscrews and the keyboard
slides out. A switch closes when you do this to take it out of circuit. Remove
three thumbscrews and the typing unit lifts out. Again, a switch closes so
this doesn't break the current loop and interfere with other machines on the
loop. Replacing the motor takes a screwdriver and about two minutes. That's
everything except the cast iron base and some switches and wiring. So a quick
swap in the field will get you going if you have the spares. It was routine to
swap these big units and take them back to a shop for cleaning and oiling once
a year or so.

At the next level of maintenance, there are removable subassemblies. The
carriage and the selector assembly, the most complicated subunits, come off
easily with tools. So they can be swapped by first-line maintenance if
necessary.

Finally, every part can be unscrewed and replaced if necessary. That's why
these machines still work. They are totally maintainable. However, they
require considerable adjustment after maintenance. Measuring spring tensions
and clearances is required, not just parts changing. Fortunately, the very
detailed manuals have survived.[4]

The price of this maintainability was the need for too much infrastructure.
Maintenance depots. Teletype mechanic schools. The military had two levels of
school - one to do installation and maintenance down to sub-assembly
replacement, and one for the shop techs who had to be able to totally
disassemble and reassemble a machine. All specialized for one or two very
specific machines. That's expensive.

[1] [https://vimeo.com/124065314](https://vimeo.com/124065314) [2]
[https://github.com/John-Nagle/ttyloopdriver](https://github.com/John-
Nagle/ttyloopdriver) [3] [https://github.com/John-
Nagle/baudotrss](https://github.com/John-Nagle/baudotrss) [4]
[http://www.aetherltd.com/manuals.html](http://www.aetherltd.com/manuals.html)

------
dbcurtis
And remember, boys and girls, that the correct sequence is CR LF, and not LF
CR. If I see LF CR in your C code I will slap you with a review comment, just
out of nostalgia, even though modern printers rarely drip oil. CR can take
more than a single character time for the carriage to move to the left and
settle, so to avoid print corruption, issue CR first, and then LF so that the
carriage also has LF time to settle.

Back many moons ago I used to operate RTTY on ham radio. On the air, since
characters can be easily lost, the standard end-of-line sequence was often CR
CR LF LF NUL NUL in case a CR or LF got clobbered by a static crash or such,
and the NULs are extra settling time. (Usually we also configured "unshift-on-
space" because a lost LTRS in Murray code is annoying. We all assumed the
other end was running U.S.O.S. so always assumed SPACE took the receiver out
of FIGS mode and therefore sent a redundant FIGS after SPACE.)

If you are wondering why UARTs can be configured for 5 data bits and 1.5 stop
bits.... Murray code as used in Model 15's is why. Once upon a time I did a
Apple II program in 6502 assembly that gave me a "glass TTY", split screen,
received text on the top 2/3's of screen, sent on the bottom. Interfaced to a
HAL ST-6000 (I loved that box. I still have it collecting dust somewhere).
Eventually I ported the glass TTY to a Macintosh I.

Now... where did I leave my cane? And get off my lawn.

~~~
frutiger
I have never seen LFCR in C or any other code (but plenty of LF and CRLF, and
since OS X, very rarely just CR). I know your comment is tongue-in-cheek but I
am genuinely curious if LFCR is otherwise common in some obscure niche.

~~~
tom_
The OSs for Acorn computers printed it in that order, as it made the 6502
version of the translating print routine (that would translate CR into LFCR) a
few bytes shorter to print them in that that order: (not exactly the code in
the ROM, but not meaningfully different)

    
    
        osasci: ; print ASCII char, preserving A, translating CR->LFCR
            cmp #13
            bne oswrch
        osnewl: ; print newline, overwriting A
            lda #10
            jsr oswrch
            lda #13
        oswrch: ; print ASCII char, preserving A
            ...
            rts
    

(According to Wikipedia, this ordering survived into the ARM range! - maybe it
helped with the ARM code too? Or perhaps they wanted the output to be
identical, or nobody thought it mattered which order you print them in... no
idea.)

CR on its own was the standard line ending, but if you saved screen output
(e.g., a BASIC listing) to a file you'd find LFCRs in the file, and if you
played the output back as input you'd see a bunch of blank lines added due to
the additional cursor movements.

I suppose this must have affected printer output, but I don't remember it
causing any issues as the printer interface had some kind of flow control.

------
dpedu
CuriousMarc, a volunteer at the Computer History Museum, recently finished a
video series restoring two Model 19 teletypes into working order. It's a long
series but eventually explains how the devices work at all levels, and shows
off some of the more niche features of the ttys. Well worth the watch.

[https://www.youtube.com/watch?v=_NuvwndwYSY](https://www.youtube.com/watch?v=_NuvwndwYSY)

~~~
teh_klev
That's a tremendous series. The mechanicals in these things are mind bending.

------
pugworthy
Another "rocking chair" story...

I grew up (60's) with a few of these clacking away in my father's HAM shack.
He was a member of the Military Affiliate Radio (MARS) program, and used to
get things like TTYs for free.

He was really open about letting me mess with things (as long as I didn't take
things apart or transmit), so I spent a lot of time playing around with them,
feeding punch tapes into them to print things, etc. I knew at the time that
punch tapes were a/the recording method for computer programs on mainframes,
so I had a lot of "this is a computer" fantasies rolling around in my brain.

------
cicero
We had one of these when I was a kid in the late 1970s. My dad built what was
essentially a modem and used it to connect the teletype to an old Motorola
single channel radio with a crystal tuned to the amateur radio (Ham radio)
teletype repeater in Dallas, Texas. There was a community of Ham radio
teletype (RTTY) users in Dallas with similar setups that formed what was
essentially a chat room. There was even one Ham that had a PDP-11 minicomputer
connected to his system that acted like a chatbot. You could send a message to
it to get weather reports and a few other interesting functions. When I would
come home from school each day, I would find several feet of paper had come
out of the teletype, and I would scan it to see what had been said while I was
gone. If I wanted to contribute and the channel was clear, I would flip the
transmit switch and start typing. Everything I typed would appear on all of
the other rigs that were connected.

We could also connect the modem to my dad's HF radio and chat with RTTY
operators all over the world. I would spin the tuning dial until I heard the
distinctive RTTY tones. I would adjust the tuner until the pitch sounded
correct and then turn on the teletype. Often garbage would come out until I
got it tuned in perfectly, and then I might see someone calling to start a
conversation, or a conversation already in progress. Sometimes I would catch a
teletype art (ASCII art using 5-bit Baudot code) picture in progress. If it
looked interesting, I would stay tuned in case they repeated the transmission.
This took a while because these machines operated at 60 words per minute (~6
characters/second).

After a while, my dad upgraded to a Model 19, which had a paper-tape punch and
reader. This allowed me to create my own teletype art, so I worked several
hours creating an R2-D2 that I was very proud of. I sent it to several people
and was thrilled one day when I was tuning around and recognized my R2-D2
being sent by someone else.

When my dad later brought home an Apple II, he was able to connect the Apple
II to the modem, so we replaced the Model 19 with a Model 28 RO (Read-only),
which did not have a keyboard or paper tape, but that was okay because we used
the Apple II. Because modern computer printers like an Epson MX-80 were rather
expensive then, we used the teletype as our printer, and I even turned in some
school papers printed on the teletype in all-caps on yellow paper torn off
from a roll. (I'm sure my teachers loved that.) When we eventually did get an
Epson printer, it was hard to justify keeping the bulky teletype, so it was
sold. We still did some RTTY now and then on the Apple II, but it wasn't quite
the same as having the teletype machine left on all day. Of course, most of
the rest of the RTTY community was also moving to microcomputers, and the
hobby began to rapidly change. Then I went off to college to study computer
science and haven't done much with amateur radio since then.

~~~
Elv13
We have this setup at my local hackspace in Montreal. Most HAM folks have not
been very active in the last few years and the Twitter API changed so the
teletype monster is now quiet again. I guess the IRC module could still work
if someone with the time and the knowledge would oil the gears. The idea at
some point was to use modified bi-directional pagers and the teletypes for a
city wide wireless comm network. It is a wonderful machine to see in motion.
Electromechanic computers are amazing.

Reminds me of other examples such as [https://arstechnica.com/information-
technology/2014/03/gears...](https://arstechnica.com/information-
technology/2014/03/gears-of-war-when-mechanical-analog-computers-ruled-the-
waves/)

