The Teletype Corporation Model 15 “typebar page printer” is a beautiful piece of equipment from the 1930’s. While the interface has much in common with modern serial communication standards – start bits and stop bits, asynchronous clocking, idle-high conditions — in the teletype all of these are implemented purely mechanically. The mainshaft looks like something out of an automobile instead of a piece of computing. Just like a car, you have to keep it well oiled, check the gaps for proper clearances and be very wary of the spinning pieces while working on it.

The 5-bit Baudot words are clocked in synchronously with the mainshaft’s rotation rate at 45.5 baud (22 ms per bit). This slow speed is difficult to generate with modern computers and serial ports, so many users bit-bang the port with Heavy Metal, which itself requires somewhat older machines to run. The 5-bit word has only thirty two entries, which is insufficient to simultaneously represent the entirety of the twenty six letters A-Z, ten digits 0-9 and punctuation. Instead there is a reserved code to switch to Figures and another code to switch to Letters. Space, carriage return and line feed are present in both the Letter and Figure sets. The interface must track which rail is currently selected and insert the correct shift sequences on the fly.

In slow motion the holding magnet can be seen pulling the selector during one-bits and not attracting it during the zero-bits. Each bit is latched via the “sword” mechanisms onto one of the rails, which select one of the twenty-eight hammers. During the fifth bit an extra gear cocks the mechanism and a latch lets the hammer fly during the stop bit. The coils are designed for 60 mA, but have a very high inductance (on the order of 4 Henries according to John Nagel). Most approaches to interfacing with the system use a large 100 VDC power supply and a current limiting resistor (that must dissipate 6 W of power!).

Both the high voltage and the custom baud rate issues are a bit cumbersome, so I thought there must be a better way. USB ports will output about 5 W, so I figured that would be close enough to drive the magnet if I could produce sufficiently high voltage. Using Adafruit’s boost converter calculator, I came up with some rough numbers and built a circuit that handles both the slow baud rate and voltage required to interface with the teletype. It shows up as a normal USB serial port and handles the 7-bit ASCII to 5-bit Baudot translation, including tracking which of the two rails is selected, outputs the bits at the correct baud rate, and runs the PWM charge pump to generate the high voltage with the inductor. A second MOSFET is used to switch the high voltage through the current loop, allowing the holding magnet to turn on and off.

It’s fully self-contained, fits in an Altoids tin and works with any software you want to run on the interface computer. If you want to run it with a Raspberry Pi, be sure to use a powered hub to avoid overloading the Pi’s weak USB power circuitry. The schematics and source code are posted to 45baud.net and you can see it in action as part of Future Crew at the digital archeology themed 2013 NYC Resistor Interactive Party.

Some technologies are so direct and intuitive that they feel classic even when they’re new. Some technologies are so ahead of their time that they only find their true purpose years after they’ve been put out to pasture.

In the early 80’s, France Telecom rolled out the Minitel, a videotex system offering various online services to users across France. Subscribers were given small, semi-portable CRT-based terminals. The service was a success, and at its peak boasted 25 million users. But eventually, well, you know. The internet. In June 2012, France Telecom finally pulled the plug on the Minitel. Screens across the country went dark. Millions of little, boxy terminals, suddenly cast adrift. Widespread technology, lost and alone, in search of purpose. Purpose now, suddenly, found.

The Minitel/Tumblr Time Tunnel is a Minitel 1B US (yes, there was a QWERTY version) backed by a Raspberry Pi. Enter a few tags at the prompt, and the mighty firehose of Tumblr will be unleashed upon your tiny, 3-bit*, 80×72 pixel black and white CRT display. By cranking the serial port up to 4800 blazin’ bits per second and reducing the number of color swaps, you can view the genius of the internet at such blinding speeds that you’ll think that you’ve suddenly been transported to a Jetsonian future of videophones and cars that collapse into briefcases. It’s just that advanced. See for yourself:

(The asterisk after “3-bit” is due to the fact that each 2×3 block of “pixels” is actually a single character with foreground and background color attributes, so each 2×3 block only has one bit of color data, selected from a palette of 8 colors.)

As is de rigueur, all the code is available on github.

The Minitel/Tumblr Time Tunnels will be on display at this year’s NYCR Interactive Party. Be sure to come by and see the internet the way it positively demands to be seen!

Coming June 15th to NYC Resistor, the 2013 Interactive Show.  We’re still soliciting for a few more projects, but we’ve already got a great menagerie of really interesting & fun work lined up.  This year’s theme is Digital Archeology.  Expect to see the old, and the old hacked into new… Our show usually sells out far in advance, so don’t dawdle. Get your tickets here: http://interactiveshow2013.eventbrite.com/

Rapid prototyping tools are great for quick hacks, but their real power lies in their ability to allow you to quickly iterate and refine a design. Earlier this week I hacked together a primitive nine-channel punched paper tape reader, but it had a number of limitations: the LEDs that I was using to read the bits were noisy and slow, the materials used didn’t mask the light well enough, the tape wasn’t mechanically aligned well, the electronics were a mess and the entire mechanism was difficult to use. This Friday, I decided to do what my third-grade teacher would tell me to do every time I half-assed something: go back and do it right.

This time I used proper phototransistors and IR LEDs I scrounged up around the space (thanks, Miria and Raphael!). Because they’re 5mm in diameter (and the spacing between channels is only 2.54mm), I had to come up with a new sensor packing. This one reads bits from four separate columns over a space of five columns, requiring an internal buffer of five columns to reconstruct a single column of data. Even so, the spacing was tight, and I had to sand down the flanges of the phototransistors and LEDs to make everything fit. I milled simple PCBs for both sides to keep things nice and neat, and used a small surface-mount potentiometer to limit the current to the LEDs in case the paper wasn’t thick enough to block enough light. The light mask is made of black acetal this time, and the spacers include runners to help keep the tape straight. There’s still no automatic feed mechanism, but we now have a reader that’s fast and reliable enough to read tapes in earnest.

The updated code, mechanical drawings, and PCB designs are all up on Github. There are still a few tweaks we’d want if we were going to scan more tapes, but this version works very well. Now we just have to figure out what to do with all these PDP-8 binaries. Any ideas?

(Note to time-travelling computer conservators: in the past/future, please do not store your paper tapes in damp basements. These programs are stinky. The Fortran compiler, in particular, is exceptionally foul. Yours truly, phooky.)

Trammell came across a cache of punched paper tape recently. My immediate impulse was to create the most primitive tape reader possible. Thusly:

The rig is composed of a Teensy++ 2.0, eighteen red LEDs, eighteen resistors, and a few bits of laser-cut plastic. LEDs are used to both illuminate the paper and sense the holes. The sensor design is based on the classic Arduino LED sensing code. It’s not very reliable, but it’s a fun afternoon proof-of-concept.

If you’re interested, the code and design files are up on github.